Electronic States and Spectra of BiO

JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.

190, 28–77 (1998)

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Electronic States and Spectra of BiO O. Shestakov, 1 R. Breidohr, H. Demes, K. D. Setzer, and E. H. Fink Physikalische Chemie-Fachbereich 9, Bergische Universita¨t-Gesamthochschule Wuppertal, D-42097 Wuppertal, Germany Received June 2, 1997; in revised form November 21, 1997

The electronic spectrum of the BiO radical has been studied by Fourier transform emission spectroscopy, laserinduced fluorescence, and excimer laser photolysis techniques. Six new electronic states, A1 ( V Å 3/2) (Te Å 11 528.8 cm01 , ve Å 530.4 cm01 , ve xe Å 2.42 cm01 ), G ( V Å 3/2) (Te Å 20 273 cm01 , ve Å 499 cm01 , ve xe Å 2.6 cm01 ), H ( V Å 1/2) (Te Å 20 469.76(6) cm01 , ve Å 471.63(18) cm01 , ve xe Å 2.153(35) cm01 ), I ( V Å 1/2) (Te Å 21 982.50(2) cm01 , ve Å 506.50(11) cm01 , ve xe Å 3.263(34) cm01 ), J ( V Å 3/2) (Te Å 25 598.95(42) cm01 , ve Å 489.95(16) cm01 , ve xe Å 2.309(45) cm01 ), and K ( V Å 1/2) (Te Å 26 744.7(2) cm01 , ve Å 420.6(4) cm01 , ve xe Å 5.25(5) cm01 ), and 14 new electronic transitions (A1 R X1 , G r X2 , H } X1 , H r A2 (A), I } X1 , I r A2 , J } X1 , J } X2 , K } X1 , K } X2 , K r A2 , B } X2 , B r A2 , C } X2 ) have been detected. Time-resolved measurements of the fluorescence decays have yielded the radiative lifetimes of the £ Å 0 levels of most states up to õ30 500 cm01 energy ( tX2 Å 480 { 100 ms, tA2 Å 9.3 { 1.5 ms, tH Å 15 { 3 ms, tI Å 16 { 3 ms, tJ Å 4.9 { 0.9 ms, tK Å 2.6 { 0.3 ms, tB Å 0.55 { 0.08 ms, tC Å 0.84 { 0.15 ms) and rate constants for quenching of the states by some rare gas atoms and simple molecules. The new electronic states A1 , G, H, I, J, and K and the previously known levels X1 , X2 , A2 (A), B, C, and D are assigned to spin–orbit states arising from low-energy valence configurations of BiO with the help of detailed theoretical data calculated by Alekseyev et al. (J. Chem. Phys. 100, 8956–8968 (1994)). q 1998 Academic Press

1600 nm, which has been identified to be the ‘‘forbidden’’ transition between the fine structure levels of the widely split 2Pr ground state of BiO (16–18). This recent work has raised some questions concerning the spectroscopy of the molecule as well as the kinetic processes leading to excitation of high-lying states of BiO in the Bix / O2 (a 1Dg ) system: (i) The first excited state of BiO was assumed to be a 2Pi state of which only the V Å 1/2 component was known (8, 19) from the prominent A(A2 ) 2P1 / 2 } X1 2P1 / 2 band system (11) (note: the state hitherto named A hereafter is referred to as A2 ). Detection of strong perturbation of the vibrational level £ Å 6 of X2 2P3 / 2 near 11 000 cm01 (18) and the observed high intensity of the perpendicular transition X2 ( V Å 3/2) r X1 ( V Å 1/2) has raised the question for the position of the A1 ( V Å 3/2) component of A and possible transitions from this state to the ground state levels or to higher-lying states. (ii) The large energy gap of 14 533 cm01 between states A2 and B brings about the problem of explaining collisional excitation of the B and C states of BiO up to energies of above 30 000 cm01 in the Bix / O2 (a 1Dg ) system. Mayo et al. (15) suggested an energy pooling mechanism involving E r V energy transfer processes with large D£ ( D£ ° 16) to populate high vibrational levels of A2 ( £ ° 16) as intermediates. Because such processes are unlikely, observation of the B r X and C r X chemiluminescence rather points to the existence of unknown electronic states in the energy range of 14 000– 28 000 cm01 . (iii) Observation of transitions between the fine structure components of widely split X2P and X3S 0 ground states of diatomic radicals recently has been reported

INTRODUCTION

Electronic emission and absorption spectra of the bismuth oxide radical have been known since 1927 (1–10). Like other group VA oxides, BiO has a 2Pr ground state arising from the rrrs 2p 4p* electron configuration. Hitherto, observation of four excited valence states (A 2P1 / 2 , Te Å 14 187.0 cm01 ; B 4S 10/ 2 , Te Å 28 738.2 cm01 ; C 2D3 / 2 , Te Å 30 700 cm01 ; D 2P1 / 2 , Te Å 32 805 cm01 ) and two Rydberg states (E 2S(?), Te Å 38 550 cm01 ; F, Te Å 40 941 cm01 ) has been reported (11, 12). In the 1960s the transitions of BiO have attracted special interest because they were among the first electronic spectra in which effects of magnetic hyperfine splitting were observed (6, 8, 13). The state of knowledge on electronic states and transitions of BiO and other group VA oxides has been reviewed by Rai and Rai (12). More recently, the molecule has found new interest for two reasons: (i) In flow-tube experiments, intense emission spectra of BiO in the near-ultraviolet, visible, and near-infrared regions have been observed from the reaction system of bismuth vapor (Bix ) with microwave-discharged oxygen (14–18). It has been concluded that vibronic levels of BiO up to energies of É30 000 cm01 are populated by multiple energy transfer and energy pooling processes in collisions with metastable oxygen molecules O2 (a 1Dg ) (15). (ii) Chemiluminescence studies in the NIR region have led to observation of an intense band system between 1300 and 1

Permanent address: Institute of Physics, St. Petersburg State University, 198904 St. Petersburg, Russia. 28 0022-2852/98 $25.00 Copyright q 1998 by Academic Press All rights of reproduction in any form reserved.

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for a number of heavy-metal containing species (20–24). In the limit of Hund’s coupling case (a) such transitions are forbidden for electric dipole radiation (25). For heavier molecules the transitions acquire some transition probability because of spin–orbit mixing of the states but they still are weak and the upper spin–orbit components are long-lived (26, 27). To assess the possible importance of these metastable molecules as energy carriers and reactants in chemical systems, information is needed on the radiative lifetimes of the states and their stability toward collisional quenching. Because of the low time response of sensitive infrared detectors, time-resolved measurements of the weak X2 r X1 emissions are difficult. Therefore it is desirable to find higherlying electronic states of the radicals, which combine with both components of the ground state. Transitions to such states can be used in LIF experiments to follow the concentrations of the radicals in both ground state levels and, provided the relative transition probabilities are known, will allow us to determine the relative yield of the states X1 and X2 in photolysis processes and chemical reactions. For BiO hitherto no such state combining with both components of the ground state has been known. To address these problems we have studied the electronic states and spectra of BiO with different experimental techniques. Emission measurements with a high-resolution Fourier transform spectrometer have been used to study in more detail the chemiluminescence spectrum from the Bix / O2 (a 1Dg ) reaction in the UV/VIS and NIR regions. LIF measurements with an excimer-laser pumped dye laser were performed to search for very weak electronic transitions and to study the radiative lifetimes and quenching of the electronic states. Excimer-laser photolysis of suitable parent compounds was used to produce electronically excited BiO radicals and to study the time decay of the emissions. Six new electronic states and at least 14 new transitions have been observed. In the present paper we give an overview of these new states and transitions and report first results of the studies with all three experimental techniques. In parallel with our experimental work, Alekseyev et al. (28) have performed detailed ab initio relativistic configuration interaction calculations of the potential curves and transition probabilities of BiO. With the help of their results, it has been possible to assign the new electronic states A1 , G, H, I, J, and K and the previously known ones X1 , X2 , A2 (A), B, C, and D to spin–orbit states arising from low-energy valence configurations of BiO. EXPERIMENTAL

The experimental techniques applied in the present work have been described in previous publications (17, 26, 27), so only brief descriptions of the experiments are given here. The chemiluminescence from the Bix / O2 ( a 1Dg ) system was studied in a fast-flow system made of pyrex glass

and quartz tubes. A flow of Ar or He carrier gas was passed over bismuth metal heated to approximately 800 7C in a quartz tube, and the Bix vapor was reacted with microwave-discharged oxygen in an observation tube of 1.4 m length and 6 cm diameter. The tube was pumped with a 500 m3 h 01 roots pump in series with a 40 m3 h 01 forepump. Spectra of the chemiluminescence were observed along the axis of the tube and were measured with a highresolution Fourier transform spectrometer ( Bruker IFS 120 HR ) . Different detectors ( Ge detector: Applied Detector Corp. Model 403 S; photomultipliers: Hamamatsu type R 928 or R 666-10 ) were used to cover the spectral range from 300 to 1750 nm. The wavenumber scale of the spectrometer was calibrated by using emission lines of the a r X and b r X bands of molecular oxygen near 1260 and 762 nm as secondary standards ( 29 ) . In the medium-resolution measurements reported here, the precision of the measured wavenumbers is on the order of one tenth of the spectral resolution used in the experiments ( 0.02 – 2 cm01 ) . The laser-induced fluorescence experiments were performed with an excimer-laser pumped dye laser ( Lambda Physik model FL 2002 E / EMG 200 or Radiant Dyes Laser Acc. model DL-MIDI E ) , which yielded pulses of É10 ns half-width, É10 mJ energy, and É0.2 cm 01 linewidth with a repetition frequency up to 10 Hz. A number of different dyes were used to cover the spectral range from 330 nm ( p-Terphenyl ) to 980 nm ( Styryl 14 ) . The cubic fluorescence cell was made of stainless steel and had a volume of É3 L. BiO radicals were produced in a fast-flow system by pyrolysis of Bi 2O3 or by reaction of bismuth vapor ( Bix ) with microwave-discharged O2 or CO2 . Helium and argon were used as carrier gases. The cell was pumped with a 250 m3 h 01 roots pump in series with a 30 m 3 h 01 forepump. The pressure in the cell was measured with a Baratron capacitance manometer and was in the range of 0.5 – 10 Torr ( 1 Torr É 133 Pa ) . The partial pressure of added quenching gas was calculated from the total pressure and the individual gas flow rates, which were measured with Tylan mass flow meters. The gas mixtures consisted mainly of He or Ar carrier gas ( É1 Torr ) and the quenching gas under study, with a few millitorrs of O2 ( CO2 ) and unknown traces of the bismuth compounds. The bismuth metal and Bi 2O3 as well as the gases had research grade purity and were used without further purification. Fluorescence was detected at right angles to the exciting laser beam. A small grating monochromator ( Spex Minimate, 22 cm focal length, gratings with 1200 ( 600 ) groves /mm blazed for 500 ( 1000 ) nm) or suitable longpass color glass and narrow bandwidth interference filters were used to measure spectra, to reduce scattered laser light or to select individual bands or band groups. Photomultipliers with a multialkali or GaAs cathode ( Hama-

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FIG. 1. Fourier transform spectrum of the chemiluminescence of BiO from the reaction of bismuth vapor with microwave-discharged oxygen. Spectral resolution: 2 cm01 ; total pressure: É5 Torr; partial pressure of discharged O2 : É 0.5 Torr; carrier gas: Ar; detectors: a-i PMT Hamamatsu R928 or R666–10, j-m Ge detector.

matsu Type R 1104 and R 666-10 ) and liquid nitrogen cooled germanium detectors ( Applied Detector Corp. Models 403 L and 403 S ) were used to measure the fluorescence light in the 400 – 900 nm and 900 – 1700 nm regions. Excitation and fluorescence spectra were recorded by use of a boxcar integrator and a strip chart recorder.

The output of the boxcar integrator was digitized with an AD converter and the data were stored in a laboratory computer. The accuracy of the absolute wavelength reading of the laser was checked by comparing the vacuum wavenumbers of excitation bands of BiO with precise data measured with the calibrated Fourier transform spec-

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FIG. 1 —Continued

trometer. The reading was found to be reliable to within an error limit of approximately {0.5 cm01 . The relative sensitivity of the monochromator / photomultiplier detection system was determined in the range of 300 – 900 nm by use of a calibrated tungsten filament lamp ( OSRAM Model Wi 41 / G ) . To measure time-resolved fluorescence decay curves, the detector signals were averaged in a

digital oscilloscope ( LeCroy Model 9400 ) . The averaged decay curves were transferred to the computer and analyzed by use of a least-squares fitting program. The photolysis experiments were performed with a small excimer laser ( Lambda Physik EMG 50 ) , which yielded pulses of KrF ( 248 nm) radiation of É30 mJ energy and É15 ns half-width at a repetition frequency

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FIG. 1 —Continued

up to 200 Hz. Bix , Bi 2O3 , or BiONO2 vapor was generated in a flow system by heating a few grams of the metal or salt in a quartz tube and was carried into the fluorescence cell by helium or argon carrier gas. The cell was pumped with a 500 m3 h 01 roots pump in series with a 40 m3 h 01 forepump. The total pressure was in the range of 0.5 – 5 Torr and consisted mainly of He or Ar carrier gas ( É1 Torr ) and the quenching gas under study. Emission of the photolysis products was measured at right angles to the exciting laser beam and the gas flow using the same detectors and techniques as for the LIF studies. ELECTRONIC STATES AND TRANSITIONS

The electronic spectrum of BiO has been investigated in four steps. First the chemiluminescence from the reaction system of Bix vapor and microwave-discharged oxygen was studied at low spectral resolution with the Fourier transform spectrometer. Figures 1a–m show the spectrum of the luminescence in the range of 5900–31 000 cm01 measured at a resolution of 2 cm01 . Different beam splitters (CaF2 , IRquartz, UV-quartz), band-pass filters, and detectors (Ge detector, photomultiplier) were used to cover this wide spectral range. Therefore the relative intensities of the bands in different sections of the spectrum may not be compared. Even within the 2000 cm01 wide sections of Fig. 1 the relative sensitivity may change considerably because of varying transmission of the filters used. Most features in the spectrum could be assigned. Besides the bands of the hitherto known transitions C r X1 , B r X1 , A2 (A) r X1 , and X2 r X1 , a number of bands are observed in the middle wavenumber range between 16 000 and 25 000 cm01 , which can readily be attributed to three new transitions to the ground state, H r X1 , I r X1 , and J r X1 . Once knowing the energies of the new states, some weak bands in the range of 16 000–19 000 cm01 could be assigned to the transition J r X2 , and a group

of bands near 6250 cm01 (Fig. 1m) was identified to be the D£ Å 0 sequence of the H r A2 transition. High-resolution spectra (0.02 cm01 ) revealed a number of weak bands with narrow lines in the range of 8000–12 000 cm01 , which were assigned to transitions from a fourth new state G to X2 . In a second step, these assignments were checked by measuring LIF excitation spectra of BiO in the entire near-UV, visible, and near-IR range. Figures 2a–n show sections of the excitation spectrum measured with different dyes and longpass filters under conditions optimized for production of a high concentration of BiO radicals in the X1 ground state (i.e., low O2 or CO2 flow rates). The spectra are not corrected for changes of the laser pulse energy and the relative sensitivity of the detection system. Transitions from low vibrational levels of the X1 2P1 /2 ground state to the states C, B, J, I, H, and A2 are readily observed. In addition, in the UV range a number of strong bands are observed, which are assigned to £* R £9 Å 0, 1 excitation bands to a fifth new state K. In the red range between 600 and 850 nm, a number of bands are found, which cannot be attributed to the A2 R X1 transition. Some of these bands are assigned to transitions to a sixth new state A1 . In a third step, vibrational levels £* Å 0 and 1 of most states were pumped by setting the laser to the heads of suitable bands, and the fluorescence spectra were scanned with the monochromator using different long-pass filters and detectors. These measurements allowed the search for very weak emissions from the pumped levels, and a number of new transitions, C r X2 , C r A1 , C r A2 , B r X2 , B r A2 , K r X2 , K r A2 , J r X2 , J r A1 (?), I r A2 and H r A2 , were detected in this way. Finally, to get more information about the nature of the transitions, some LIF excitation bands were measured at maximum resolution of the pulsed laser (0.04 cm01 with and 0.2 cm01 without intracavity etalon), and some sections of the chemiluminescence spectrum were measured at medium or

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high resolution (0.2–0.02 cm01 ) with the FT spectrometer. This work has not been completed yet, and no detailed results of high-resolution analyses are reported in the present paper. In the following sections we will discuss the state of knowledge on the different electronic states and transitions of BiO as obtained from literature work and from the present studies. We start with the known states X2 , A2 , B, and C and their emission spectra, which have yielded the basic data for the ground state of the molecule. X2 2P3 / 2 (X2 2P3 / 2 r X1 2P1 / 2 )

The X2 2P3 / 2 state is the most recent among the known states of BiO. By extrapolation of the X 2Pr fine structure splitting down the series NO, PO, AsO, SbO, BiO ( 4, 8) and comparison with the ground state splitting in the isoelectronic group IV halides (4), the X2 2P3 / 2 state had been estimated to lie between 8000 and 10 000 cm01 , but until recently no transitions involving this state had been observed. Mayo et al. (15), in a Fourier transform study of the emissions of electronically excited BiO obtained by energy transfer from metastable oxygen molecules, observed a spectrum in the 1400–1600 nm region, which they assigned to previously unobserved low-lying V Å 1/2 and V Å 3/2 states of BiO. Shortly later, Fink et al. (17) reported on similar experiments and observation of the same bands in the NIR region. They assigned two strong band series near 7080 and 6400 cm01 to the D£ Å 0 and D£ Å 01 sequences of the ‘‘forbidden’’ transition X2 2P3 / 2 r X1 2P1 / 2 between the fine structure components of the ground state of BiO and deduced first vibrational constants for the X2 state. In a more detailed study of these emissions, Linton et al. (18) improved the vibrational analysis of the system by choosing different positions of the band origins and including the D£ Å /1 sequence in the analysis. Shifted bands observed at the lowfrequency sides of all three sequences were attributed to perturbed £* Å 6, £9 bands of the transition. However, the authors did not identify the perturbing state and also did not discuss correctly the structures of the bands in the different sequences. As is seen from Fig. 1m, the X2 r X1 transition yields by far the strongest emissisons in the NIR range. Three sequences of bands are observed, which are assigned to D£ Å 0, D£ Å 01, and D£ Å /1 transitions. The structures of the sequences were found to depend on the nature and pressure of the carrier gas. With Ar at low total pressure, a hot vibrational distribution in the upper state is observed whereas with He and at high pressure the vibrational population is much more relaxed (Figs. 3–5). Series of regularly spaced bands are observed in the D£ Å 0 and D£ Å /1 sequences, which can be assigned to the vibrational transitions 0–0 to 5–5 and 1–0 to 5–4. In all three sequences, additional bands are observed near the expected positions of the £* Å 6, £9 transitions, which do not fit into the series neither from

their shapes nor from their intensities. In accord with the suggestion of Linton et al. (18), these features will be explained by a perturbation. The bands in the D£ Å 0, D£ Å 01, and D£ Å /1 sequences show quite different shapes (Figs. 3a, 4a, and 5). The structure of the bands is easily conceived from the relaxed spectra measured with He carrier gas (Figs. 3b and 4b). Because of the large V-doubling in the X1 2P1 / 2 ground state ( Dnfe Å 0.187(J / 1/2) cm01 (8)), the bands consist of two R, Q, and P branches each. Indeed, like in the analogous spectrum of TeH (20), the two Q branches are well separated and degraded in opposite directions. Figure 6 shows a simulated spectrum of the 0–0 band calculated for a rotational temperature of 300 K with the linestrengths formula given in Ref. (25) and rotational constants given in Table 1. In all simulated spectra, for the ground state the constants Be , ae , and p£ were taken from Barrow et al. (8), and for the X2 state rotational constants were used, which were derived from analyses of high-resolution spectra of the new G r X2 and J r X2 transitions (see below). The hyperfine structure splitting of the lines was ignored. As is seen from the simulation, the strongest feature in the 0–0 band is the Q f branch, which forms a head. The second maximum is mostly caused by the Qe branch, the Re , R f and Pe , P f branches are broader and therefore, at low resolution, show much lower intensity (Fig. 6b). The origin of the band lies between the two maxima, as was assumed by Linton et al. (18) who assigned the second maximum to the R branches. The other bands of the D£ Å 0 sequence and those of the D£ Å 01 sequence have similar shapes (Fig. 7a). As discussed by Linton et al. (18), the appearence of the bands is typical for a transition in which the upper and the lower states have very similar potential curves and consequently show nearly identical vibrational and rotational constants. In such a case, the anharmonicity terms play an important role for the structure of the bands and band sequences. In the case of the X2 r X1 transition of BiO, this leads to two peculiarities: (i) Because ( v e* 0 v *e x *e ) ú ( v e9 0 3ve x e9 ) and ve x *e ú ve x 9e ,, for the D£ Å 01 sequence, in the expression for the band wavenumbers in Eq. [1], the terms linear and quadratic in £ have different signs. Because the linear term is small, the quadratic term causes the bands to form a ‘‘head of heads’’ (18) and turn around (Fig. 4a). n£ r £/1 Å T *£ 0 T 9£/1 Å Te / [1/2( v *e 0 3v e9 ) 0 1/4( ve x *e 0 9ve x 9e )] / [( v *e 0 ve x *e ) 0 ( v 9e 0 3ve x 9e )] £ 0 [ ve x *e 0 ve x 9e ] £ 2

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FIG. 2. LIF excitation spectra of BiO measured with argon carrier gas under conditions of maximum concentration of BiO in the X1 2P1 / 2 ground state. Pressure of Ar carrier gas: 1.5 Torr; pressure of CO2 : É1 mTorr. The following dyes, Schott color glas filters, and Hamamatsu photomultipliers were used, a: p-Terphenyl, KV 370, R1104; b: DMQ, GG 395, R1104; c: Polyphenyl 2, KV500, R1104; d: PBBO, KV 500, R1104; e: Coumarin 120, GG 495, R1104; f: Coumarin 102, KV 550, R1104; g: Coumarin 307, RG 610, R666-10; h: Coumarin 153, RG 630, R666–10; i: Rhodamin B, RG 665, R666–10; j: Sulforhodamin 101, RG 715, R666–10; k: Pyridin 1, RG 780, R666–10; l: Pyridin 2, Si filter, Ge detector; m: Pyridin 4, Si filter, Ge detector; n: Styryl 9M, Si filter, Ge detector.

( ii ) The different forms of the Q branches in the D£ Å 0 and D£ Å /1 bands are explained by the small difference between the Be values in both states compared with

the anharmonicity constants a e* É a e9 Å ae resulting in B e* 0 B e9 ú 0 but B *e 0 ( B 9e / ae ) õ 0. Thus in the D£ Å /1 sequence the second and third terms in Eq. [ 2 ] )

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FIG. 2 —Continued

change signs, which leads to a change of the shading of the two Q branches.

D£ Å /1:

nQ (J)e , f Å n0 / (B *e 0 (B 9e / ae ))J / (B *e 0 (B 9e / ae ))J 2

nQ (J)e , f Å n0 / (B *£ 0 B 9£ )J / (B *£ 0 B 9£ )J 2

{ (p£ 0 /2)(J / 1/2).

[2]

{ (p£ 0 /2)(J / 1/2)

D£ Å 0:

nQ (J)e , f Å n0 / (B *e 0 B 9e )J / (B *e 0 B 9e )J 2 { (p£ 0 /2)(J / 1/2)

The simulated spectrum of the 1–0 band calculated with the constants given in Table 1 (Fig. 7b) is in good agreement with the experimental shapes of the D£ Å /1 bands (Fig.

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FIG. 2 —Continued

5). Again the origins of the bands lie between the two Qbranch maxima. In the spectrum measured at low pressure with Ar carrier gas, the intensities of the bands in the D£ Å /1 sequence show an unexpected systematic increase by about a factor of 6 from the 1–0 to the 5–4 band (Fig. 5). Since the Franck–Condon factors of all D£ Å 0 bands can be assumed to be large and similar in magnitude (qD£Å0 É 0.9), the relative vibrational populations in the X2 state can be estimated from the relative intensities of the 0–0 to 5–5 bands. They are found to correspond to a Boltzman distribution at about 1 800 K, i. e., the population in level £* Å 5 is about a factor of 13 smaller than that in £* Å 1. Therefore, to explain the intensity pattern in the D£ Å /1 sequence, the band transition probabilities A£ =£ 0 must increase by about a factor of 80 from the 1–0 to the 5–4 band. A similar, but weaker increase of A£ =£ 0 is observed in the D£ Å 01 sequence. As was suggested by Linton et al. (18), the additional features observed in the D£ Å 0, D£ Å /1, and D£ Å 01 sequences are assigned to perturbed £* Å 6, £9 bands. The perturbing state obviously is the new state A1 ( V Å 3/2) observed in the LIF experiments (see below). Avoided crossing of the two V Å 3/2 potential curves leads to a widening of the X2 2P3 / 2 curve and a lowering of the £* Å 6 state energy (28). As a result, the £* Å 6, £9 bands are shifted to lower wavenumbers by about 16 cm01 and have smaller B * values. Figures 8a,b and 9a,b show simulations of the 6–5, 6–6, and 6–7 bands calculated with the B * value deduced from analysis of the 0–6 band of the G r X2 system (Table 1). The shapes of the simulated bands agree quite well with the experimental findings. The origin of the 6–6 band lies at the low wavenumber side of the sharp Qe branch at 7 019.6 cm01 . Using the DG£/1 / 2 values of the ground state (16), the origins of the 6–5 and 6–7 bands are calcu-

lated to lie at 7660.68 and 6387.11 cm01 , in good agreement with the experimental values (Table 2). Weak features observed in the FT spectrum at 8309.8 and 8968.0 cm01 (Fig. 1l) can be identified as the 6–4 and 6–3 bands. No other bands of the D£ Å /2 and D£ Å /3 sequences are observed showing that the perturbation leads to enhancement of the £* Å 6, £9 band intensities. Similar perturbations have been observed in the X2 r X1 spectra of BiS and BiSe (30). The origins of 18 bands read from the spectra (Figs. 3– 5) with a relative accuracy of É0.5 cm01 are given in Table 2. The data were used to calculate vibrational intervals DG£/1 / 2 for levels £* Å 0–5 of the X2 state. Some more accurate DG£/1 / 2 values for this state were obtained from bandheads in the G r X2 and J r X2 systems (see below). The DG£/1 / 2 data were fitted using the expression for anharmonic oscillators [3] DG£/1 / 2 Å G( £ / 1) 0 G( £ ) Å ve[( £ / 3/2) 0 ( £ / 1/2)] 0 ve xe[( £ / 3/2) 2 0 ( £ / 1/2) 2 ] / ve ye[( £ / 3/2) 3 0 ( £ / 1/2) 3 ].

A closer look at the data indicates that also the £* Å 5 level is somewhat shifted by the homogeneous perturbation by the new V Å 3/2 state. Therefore, only the first four vibrational intervals were fitted and, as for the X1 component of the ground state the term with ve ye is found to be small (see below), only terms with ve and ve xe were used. The vibrational constants obtained from the fit (Table 3) are in good agreement with the results of Linton et al. (18). Finally, the electronic energy of the X2 state was deduced from a fit of the X2 r X1 bands in which the vibrational constants of both states were fixed to the values obtained from the fits of

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FIG. 3. Spectrum of the D£ Å 0 sequence of the X2 2P3/2 r X1 2P1/2 transition of BiO at a resolution of 0.5 cm01 . a: Ar carrier gas; b: He carrier gas.

vibrational intervals DG£/1 / 2 , and Te only was allowed to vary. This procedure allowed us to include data from different electronic transitions in the fits of vibrational intervals thus averaging errors due to changes of the separation between band origins and band heads inherent in vibrational analyses based on bandhead data. For this reason this procedure was applied throughout this work. Figures 10a,b show a high-resolution (0.007 cm01 ) spectrum of the 0–0 band of the X2 r X1 transition measured with He carrier gas. As has been observed before (15, 17), at higher J values all lines are split into 10 hyperfine components (Fig. 10b). The width of the line groups is about 0.52

cm01 and is found to be nearly independent of the rotational quantum number J and the V-doubling state. This shows that the splitting arises mostly from electron spin–nuclear spin interaction (hyperfine doubling (31)) which, in case (a) coupling, gives a first-order effect only for 2P1 / 2 but not for 2P3 / 2 states (31). Therefore the width of the line groups can be attributed to h.f.s. in the X1 2P1 / 2 ground state. For large J values, the width of the line groups approximates the product of the splitting parameter d and the nuclear spin quantum number I Å 4.5 (31). The calculated splitting constant dX 1 Å 0.115 cm01 is larger than the value previously deduced from the hyperfine splitting of the lines in A2 r X1

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FIG. 4. Spectrum of the D£ Å 01 sequence of the X2 2P3 / 2 r X1 2P1 / 2 transition of BiO at a resolution of 0.5 cm01 . a: Ar carrier gas; b: He carrier gas.

bands (8, 13, 16). Because of the large number of lines, which are heavily overlapping in the center of the band, a rotational analysis of the band will be laborious. On the other hand, because there are no heads in the R and P branches and the position of the origin is rather well known from analysis of the new band systems (see below), the J assignment of the P and R lines in this band is easier and more reliable than in bands of other transitions, which show heads in the R branches (e.g., the A2 r X1 system). Therefore a rotational analysis of the band is in progress. Direct laser excitation of the low-lying vibrational levels of the X2 state (e.g., with a Raman-shifted dye laser) via the

X2 R X1 , D£ Å 0, {1 transitions has not been attempted. However, X2 r X1 fluorescence was observed following laser pumping of all higher-lying states of BiO. The strongest interaction of the X2 and A1 potentials occurs in the region of X2 , £ Å 6 and causes this level to be a bottleneck for collisional relaxation from higher-lying states to X2 , £ ° 6 and X1 . At low pressure of Ar carrier gas, the vibrational population in X2 is found to be hot, the D£ Å 0 sequence is broad and consists mostly of bands from vibrational levels X2 , £ Å 6, 5, and 4. At the addition of D2 , the vibrational population in X2 is strongly relaxed. Hence the ratio of intensities of the D£ Å 01 and D£ Å 0 sequences decreases, the

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FIG. 5. Spectrum of the D£ Å /1 sequence of the X2 2P3 / 2 r X1 2P1 / 2 transition of BiO at a resolution of 0.5 cm01 measured with Ar carrier gas.

D£ Å 0 sequence becomes more narrow and its center is shifted to lower wavelengths. All observations indicate that the fluorescence now originates mostly from low vibrational levels £ Å 0, 1, and 2 of X2 . Fluorescence of the X2 r X1 bands of BiO also was observed from KrF excimer-laser photolysis of Bi 2O3 and BiONOz vapors. The 248-nm photolysis of BiOCl unexpectedly did not yield BiO emission but intense NIR luminescence of the X21 r X10 / bands of BiCl (23). Strong emission of the X2 r X1 bands of BiO was observed following KrF laser photolysis of Bix vapor in the presence of O2 , CO2 , SO2 , NO2 , or N2O. In this case, the excited BiO molecules likely are produced by reactions of electronically excited bismuth atoms or molecules, e.g.,

Bi 2 (Bix ) / hn (248 nm) r Bi* / Bi(Bix 01 ) Bi 2 / hn (248 nm) r Bi * 2

has been observed, which extended over the near-ultraviolet, visible, and near-infrared wavelength ranges (1–3, 5, 7). Later the system was also measured in absorption (6, 8), and rotational analyses of a number of bands have been performed. Each band was found to consist of four branches, Re , Rf , Pe , and Pf (named Rd , Rc , Pd , and Pc in (8)), pointing to large V doubling in one or both states. Because no evidence of even weak Q branches were found, Barrow et al. (8) concluded that the bands are caused by a transition between the V Å 1/2 components of two 2P states. The lower state was identified to be the X1 2P1 / 2 substate of the X2Pr TABLE 1 Spectroscopic Constants Used for the Simulation of Bandshapes of X2 r X1 Bands (cm01 )

[4a] [4b]

Bi* / O2 r BiO*(X2 , H, rrr) / O

[5a]

Bi * 2 / O2 r BiO*(X2 , H, rrr) / BiO

[5b]

Both techniques, excimer-laser photolysis and dye laser pumping via higher-lying electronic states, yielded X2 r X1 emission intensities high enough for time-resolved measurements of the fluorescence decays and studies of the radiative lifetime of the state and of quenching rate constants (see Chapter 4). a

A2 (A) ( V Å 1/2)(A2 } X1 2P1 / 2 )

From (8): B£ 0 Å 0.3034 0 0.0022 ( £9 / 1/2). Calculated from Kratzer’s relation De Å 4B 3e / v 2e ; fixed for all values of £9. c From (8), fixed for all values of £9. d From analyses of the 0–0 band and of G r X2 and J r X2 bands (Table 13). b

In early studies of the emission of BiO from arc discharges between bismuth electrodes and from a high-frequency discharge in BiOCl vapor, a main system of red-degraded bands

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curves of the A2 (A) and X1 states and of Franck–Condon factors of the A2 (A) r X1 bands (9, 32). Recently, the A2 r X1 system of BiO has been studied in the range 7550–20 000 cm01 via high-resolution Fourier transform spectroscopy by Martin et al. (16). In a vibrational analysis, 52 bandheads covering vibrational levels £* Å 0– 9 and £9 Å 0–15 were fitted and vibrational constants of both states were obtained. The rotational constants deduced from analyses of nine bands were found to be in good agreement with the results of Barrow et al. (8). Because of the higher resolution attainable in their experiments, Martin et al. (16) could resolve and analyze the hyperfine structure of some lines. They attributed the splitting of the lines to magnetic hyperfine doubling (31) in the X1 2P1 / 2 state and deduced a splitting constant of dX 1 Å 0.0928(4) cm01 (called b in their work). Figures 1 and 2 show that the A2 } X1 transition yields the most prominent and extended system of red-degraded bands in the chemiluminescence and LIF excitation spectra. For vibrational analysis, the FT spectrum was measured at a resolution of 0.5 cm01 , and the vacuum wavenumbers of the heads of 83 bands were measured with an accuracy of {0.2 cm01 . The data covering vibrational levels £* Å 0–13

FIG. 6. Computer simulation of the 0–0 band of the X2 2P3/2 r X1 2P1/2 transition of BiO at spectral resolutions of 0.03 cm01 (a) and 0.5 cm01 (b).

ground state of the molecule arising from the s 2p 4p* electron configuration. At that time the X2 2P3 / 2 component of this state was not known yet, but was estimated to lie at approximately 8000 cm01 . The observed splitting of the branches was found to be mostly due to large V doubling in the X1 2P1 / 2 state. The upper state of the transition was named A 2P1 / 2 and was thought to be the V Å 1/2 component of a second 2Pr state regarded as being one of the states emerging from the configuration s 2p 3p* 2 . The V Å 3/2 component of this state has not been observed. Later work on the analogous A 2P r X 2Pr transition of SbO (19) has shown that in this molecule the A 2P state is inverted with a spin–orbit splitting constant of A Å 0674 cm01 . By extrapolation of this splitting down the series of group V oxides, then it could be concluded that the first excited 2P state of BiO likewise should be inverted and should show a splitting of approximately 1500–2000 cm01 . The rotational lines of the A2 (A) r X1 transition of BiO were found to be broad with a width of about 0.4 cm01 , some 20 times that of the expected Doppler width (6, 8). This large linewidth was attributed to unresolved nuclear magnetic hyperfine splitting in the X1 2P1 / 2 ground state (8, 13). The spectroscopic constants determined by Barrow and co-workers later have been used to calculate RKR potential

FIG. 7. Computer simulations of the 0–1 (a) and 1–0 (b) bands of the X2 2P3 / 2 r X1 2P1 / 2 transition of BiO at a spectral resolution of 0.5 cm01 .

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four branches, Re , R f , Pe , and P f , with the R branches forming heads at rather low values of J. For higher J values, the groups of hyperfine structure lines are well resolved (Fig. 11b). But as the position of the band origin is not known, the correct J assignment is difficult. First attempts of analysis by least-squares fits of all lines showed that the J assignment could be changed by up to two units without much change in the standard deviation of the fit. Therefore, analysis of the X2 r X1 , 0–0 band, and of bands of other transitions will be helpful for correct J assignment in the different vibrational levels of the ground state. The width of the groups of hyperfine lines in the 2–10 band is found to be approximately 0.42 cm01 , i.e., substantially smaller than in the X2 r X1 bands (Figs. 10b and 11b). The width is similar for the Re , R f , Pe , and P f lines and nearly independent of J, indicating that the splitting is caused by magnetic hyperfine doubling (31) in 2P1 / 2 states. The different widths of the line groups in the X2 r X1 and A2 r X1 bands thus shows that some splitting also occurs in state A2 . In A2 the splitting constant dA 2 has the same sign as in the ground state such that the splitting effects are partly cancelling each other. The dA 2 is calculated to be É0.022

FIG. 8. Computer simulation of the perturbed 6 – 5 band of the X2 2P3 / 2 r X1 2P1 / 2 transition of BiO at spectral resolutions of 0.03 cm01 (a) and 0.5 cm01 (b).

and £9 Å 0–16 are given in Table 4. When cubic terms with ve ye were included, the wavenumbers of 78 bands could be fitted to the anharmonic oscillator expression ( 25) with a standard deviation of 0.12 cm01 . Vibrational constants of the X1 and A2 states also were deduced from fits of the vibrational intervals DG£/1 / 2 calculated from the band wavenumbers in Table 4 to Eq. [3]. In the case of the X1 ground state, DG£/1 / 2 data obtained from other band systems (H, I, J, B, C r X1 ) were included in the fit to average the errors inherent in a vibrational analysis based on bandheads rather than band origins. The 101 measurements covering spacings up to DG13.5 could be fitted with a standard deviation of 0.09 cm01 . For the A2 state, 56 data covering vibrational separations up to DG12.5 were fitted with a standard deviation of 0.11 cm01 . Within the error limits, the vibrational constants obtained in this way were identical to those obtained from the fit of the A2 r X1 bands. The results are given in Table 3. For both states, the constants are in close agreement with the data given by Martin et al. (16). Figures 11a,b show a section of a spectrum (2–10 band) measured at high resolution (linewidth 0.022 cm01 ). As is known from previous work (8, 16), the bands consist of

FIG. 9. Computer simulations of the perturbed 6–6 (a) and 6–7 (b) bands of the X2 2P3 / 2 r X1 2P1 / 2 transition of BiO at a spectral resolution of 0.5 cm01 .

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TABLE 2 Experimental and Calculated Band Origins of the X2 2P3 / 2 r X1 2P1 / 2 Bands and Vibrational Intervals DG£/1 / 2 in the X2 State (cm01 )

at least in part, to the X2 state. Laser pumping of A2 followed by fast electronic relaxation has been used in time-resolved studies of the X2 state. The positions of the A2 r X2 bands have been calculated and a search for such bands has been made in the Fourier transform spectra and in the fluorescence spectra on laser pumping of low vibrational levels of A2 . No emissions that definitely can be assigned to A2 r X2 bands have been found showing that the transition probability of this subtransition is by at least a factor of 50 lower than that of the A2 r X1 system. Weak emission of A2 r X1 bands has been observed in the region 600–850 nm on 248 nm KrF laser photolysis of Bi 2O3 and BiONO2 vapor, but the intensities were too low for time-resolved measurements. B ( V Å 1/2)(B } X1 , B } X2 , B r A2 )

The bands hitherto attributed to the B 4S 10/ 2 } X1 2P1 / 2 and C D3 / 2 } X1 2P1 / 2 systems of BiO first have been observed by Bridge and Howell (4). In absorption spectra, they found two series of red-degraded bands with well-defined heads ˚ region. The bands fitted in a De´slandres in the 3500 A scheme for a doublet system with a separation Dne of approximately 1480 cm01 and with similar vibrational constants ve É 487 cm01 for both components. As the transitions to, or from, the 2P1 / 2 ground state appeared to be of the form DV Å 0 in this heavy molecule, i.e., V Å 1/2 } V Å 1/2, Bridge and Howell assigned the bands to two separate case (c) V Å 1/2 upper states. Babu and Rao (7) observed the same transitions in the emission spectrum of a highfrequency discharge in BiOCl vapor and measured highresolution spectra of four bands of the £* Å 0, £9 progression from the lower one of the two V Å 1/2 states. They found the bands to consist of only two branches P and R, and, following a suggestion of Scari (5), assumed the upper state to be the V Å 1/2 component of a second 2P state arising from the same s 2p 3p* 2 electron configuration as the A2 (A) state. In a high-resolution study of the absorption spectrum of BiO, Barrow et al. (8) analyzed the 2–0, 3–0, 4–0, and 5–0 bands of this transition. They found the bands to contain, in addition to the two strong branches observed by Babu and Rao (7), two additional somewhat weaker branches. Also, a systematic search of combination differences in the ground state revealed no values of D2 F 9e and D2 F 9f , which were known from the analysis of the A2 (A) r X1 system. Instead, two equal ground state differences D1 F 9ef were observed. From this fact and the peculiar energy level pattern (p É 4B) and relative intensities of the four branches, Barrow et al. concluded that the upper state was the V Å 1/2 component of a 4S 0 state they named B 4S 0 . Mayo et al. (16) observed a number of B r X1 bands in the chemiluminescence spectrum emitted from the reaction of Bix vapor with metastable oxygen molecules, but they did not give the wavenumbers of the observed bands, 2

a Numbers in parentheses give the obs-calc differences from the fit with fixed vibrational constants in units of the last digit. b Not used in the fits. c From J r X2 bands. d From G r X2 bands.

cm01 , i.e., about a factor of 5 smaller than dX 1 . Cancelling of hyperfine splitting effects has also been observed in the D 2P1 / 2 R X1 2P1 / 2 system of BiO (8). As is seen from the laser excitation spectra in Fig. 2, with the pulsed dye laser all vibrational levels of A2 up to £* Å 16 can be pumped with good efficiency. Figures 12a,b show LIF spectra measured when the £* Å 0 and £* Å 1 levels were pumped via the 0–0 and 1–1 bands. In the photomultiplier range, the spectra contain only the £* Å 0, £9 and £* Å 1, £9 progressions of fluorescence bands. Vibrational relaxation by the carrier gases (Ar and He) and the small partial pressures of O2 or CO2 needed for the production of ground state BiO radicals is found to be negligible (Fig. 12b). Using suitable band-pass filters, the emissions can readily be used for lifetime and quenching measurements of both vibronic states (see Chapter 4). In the Ge detector region, strong emission of the X2 r X1 bands is observed showing that collisional quenching of the A2 state proceeds,

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TABLE 3 Electronic Energies and Vibrational Constants of Electronic States of BiO (cm01 )

and, as is shown below, their spectra measured at low spectral resolution with a monochromator did not allow correct identification of all bands. As is seen from Figs. 1a–d, the Fourier transform spectrum of the chemiluminescence contains mainly the £* Å 0, £9 and £* Å 1, £9 progressions of the B r X1 system. The last bands observed in the £* Å 0, £9 and £* Å 1, £9 progressions are 0–6 and 1–8. The bands attributed to 0–8, 0–9, 0–10, 1–10, and 3–8 by Mayo et al. (16) obviously belong to the new J r X1 and I r X1 systems. Weak features observed at 29 569.7, 28 886.9, 27 546.81, and 30 019.81 cm01 are attributed to the 2–0, 2–1, 2–3, and 3–0 bands. The shapes of the £* Å 2, £9 bands are different from those of other B r X1 bands indicating that level B, £* Å 2 is strongly perturbed. The wavenumbers of the heads of the observed B r X1 bands are given in Table 5.

In the LIF excitation spectrum (Figs. 2a,b), bands from low vibrational levels of the ground state ( £9 Å 0–2) to levels £* Å 0–2 of the B state are observed. Figures 13a,b show fluorescence spectra measured at low resolution with the monochromator when the level B, £* Å 0 was pumped with the dye laser via the 0–0 band. The strongest features in the spectra are the B r X1 bands, which can be followed up to 0–10. The relative intensity within the progression roughly agrees with that observed in the Fourier transform spectrum. Besides the B r X1 bands, weak bands of the hitherto unknown B r X2 and B r A2 systems are observed (Figs. 13b and 14a,b). The relative intensities of the B r X1 , B r X2 , and B r A2 bands were measured under controlled conditions (laser energy, flow conditions, and time window of the boxcar integrator) with the calibrated detection system. The results are given in Table 6 together with band

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FIG. 10. High-resolution spectrum of the 0–0 band of the X2 2P3 / 2 r X1 2P1 / 2 transition of BiO (a). The line width is 0.0075 cm01 . b: Section of the R branches showing the h.f.s. splitting of the lines.

transition probabilities A£ =£ 0 calculated from the relative band intensities and the measured lifetime of the B state. The ratios of the electronic transition probabilities of the B r X1 , B r X2 , and B r A2 systems are found to be 25:1:1. Both, the B r X2 and B r A2 systems, are too weak to be observed in the FT spectra. Vibrational constants for the B state were obtained from a fit of observed vibrational spacings DG£/1 / 2 to Eq. [3]. Values of DG1 / 2 , DG3 / 2 , and DG5 / 2 were deduced from our FT spectra and data for DG5 / 2 , DG7 / 2 , and DG9 / 2 were taken

from the work of Barrow et al. (8). As is seen from Table 5, the DG7 / 2 and DG9 / 2 data deviate from the straight-line fitted to the first three vibrational separations. This finding agrees with the note of Barrow et al. (8) that in state B the vibrational levels converge more rapidly than is expected by terms in only ve and ve xe . The behavior is quite similar to that found in the X2 state and suggests that the higher vibrational levels of the B state likewise are shifted due to an homogeneous perturbation by some other state of the same species ( V Å 1/2). The Te value of the B state was deduced

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TABLE 4 Bandheads of the Observed Bands of the A2 r X1 2P1 / 2 Transition of BiO (in cm01 )

45

state. This finding suggests that the levels of state B do not exhibit substantial hyperfine splitting. In spite of their lower transition probabilities, a number of LIF excitation bands of the B R X2 transition were readily observed in the 434–500 nm region (Figs. 16a,b) when a high concentration of BiO radicals in the X2 state was generated by use of a higher flow and pressure of discharged oxygen and the fluorescence was measured in the 300–400 nm range with color-glass filters (Schott UG 11 and BG 12). Figure 17 shows a medium-resolution (0.15 cm01 ) excitation spectrum of the 0–0 band of the B R X2 transition. The band likely consists of R, Q, and P branches, and the lines do not show h.f.s. broadening. No doubling of the branches due to L doubling in state B is observable. C ( V Å 3/2)(C } X1 , C } X2 , C r A1 , C r A2 )

from a fit of the bandheads (Table 5) with fixed vibrational constants. The results obtained from the fits are given in Table 3. Figures 15a,b show a medium-resolution (0.1 cm01 ) spectrum of the 0–2 band. The structure of the band and the number of branches is not discernable at the present spectral resolution and S/N ratio. At a first sight the band seems to consist of one R and one P branch only with the R branch forming a head at low J values, as was found by Babu and Rao (7). The width of the features shown in Fig. 15b is approximately 0.58 cm01 , i.e., close to the value expected from folding of the resolution of the spectrometer (0.1 cm01 ) and the h.f.s. splitting of 0.52 cm01 in the X1 ground

State C is the upper state of the second weak band system ˚ region. observed by Bridge and Howell (4) in the 3500 A It was named C by Barrow et al. (8) who found a single broad-line red-degraded band belonging to this system in their high-resolution absorption spectrum (1–0 band of the De´slandres scheme shown in (4)). The band consisted of six branches, Re , Qe , Pe , and R f , Q f , P f , and the L-type doubling in the upper state was very small. These findings led the authors to conclude that state C has V Å 3/2. Considering that transitions in which DS Å 0 usually are rather strongly forbidden, they suggested state C to be the V Å 3/2 component of a 2D state arising from the same lowlying sp 4p* 2 configuration as B 4S 0 . Mayo et al. (16) observed some bands of the C r X1 transition in the emission spectrum resulting from reaction of Bix vapor with metastable oxygen but did not give wavenumbers of the bands. As is seen from Figs. 1a–c, the FT spectrum of the Bix /O * 2 chemiluminescence contains a number of bands of the £* Å 0, £9 and £* Å 1, £9 progressions of the C r X1 transition. No bands from higher vibrational levels of the C state are observed. The vibrational assignment of the bands agrees with that of Bridge and Howell (4) and is further supported by the results of LIF measurements. In the wavelength range accessible with the undoubled dye laser (down to 330 nm), some LIF excitation bands of the C R X1 transition are readily observed (Figs. 2a-b) but again only vibrational levels £* Å 0 and 1 are reached. Figs. 18a,b show the fluorescence spectrum measured when level C, £* Å 0 was pumped in the 0–1 band. Besides the fluorescence series of C r X1 bands, the £* Å 0, £9 progression of the new C r X2 transition shows up with rather high intensity. Some additional weak bands are observed near 540 and 630 nm (Figs. 18a,b), which do not fit into the series of C r X2 bands. They are tentatively identified to be C r A1 and C r A2 transitions. The wavenumbers of the heads of the bands observed in the FT spectrum are given in Table 7. From the separation

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FIG. 11. High-resolution spectrum of the 2–10 band of the A2 4P1 / 2 r X1 2P1 / 2 transition of BiO (a). The linewidth is 0.02 cm01 . b: Section of the R branches showing the h.f.s. splitting of the lines.

of £* Å 0, £9 and £* Å 1, £9 band pairs the average value of DG1 / 2 is found to be 480.13 { 0.2 cm01 . Approximate values of the vibrational constants ve and ve xe for the C state were obtained by fitting our DG1 / 2 data and the less accurate values of DG3 / 2 , DG5 / 2 , and DG7 / 2 calculated from the band wavenumbers given by Bridge and Howell (4) (Table 7). Finally, an approximate value of Te was obtained by fitting the wavenumbers of the £* Å 0, £9 and £* Å 1, £9 bands with fixed vibrational constants. The results are given in Table 3.

The relative intensities of the C r X1 and C r X2 bands were measured under controlled conditions and, together with the measured lifetime of the C state, were used to calculate Einstein coefficients A £ =£ 0 of the bands ( Table 8 ) . The ratio of the transition probabilities of the C r X1 and C r X2 transitions is found to be 2.0. Because of their similar transition probabilities these systems should be better suited for monitoring BiO radical concentrations in the X1 and X2 states by LIF than the B } X1 , X2 systems. Excitation bands of the C R X2 transitions are readily

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FIG. 12. LIF fluorescence spectra of the £* Å 0, £9 and £* Å 1, £9 progressions of the A2 4P1 / 2 r X1 2P1 / 2 transition of BiO at a resolution of É5 nm.

observed in the 434 – 512 nm region ( Fig. 16 ) under the same conditions as used for measuring the B R X2 bands. Figure 19 shows a LIF spectrum of the C r X1 and C r X2 bands when level C , £ * Å 0 was pumped in the 0 – 4 band of the C R X2 system. In high-resolution FT spectra, the C r X1 bands became very weak and nearly disappeared in the noise. This behavior was found to be characteristic for weak bands consisting of lines, which, at higher resolution, split in many h.f.s. components. Most bands of the C r X2 transition are superimposed by strong bands of other systems and, therefore,

could not be measured at high resolution. Figs. 20a,b show a spectrum of the 1–1 band measured at a resolution of 0.05 cm01 . The width of the lines is õ0.06 cm01 showing that the rotational levels of state C do not exhibit hyperfine splitting. Therefore, the C r X1 lines display the full h.f.s. of the ground state. Medium-resolution laser excitation spectra of C R X2 bands show simple structures. Likely, the bands consist of one R and one P branch only. Both findings give support to the assumption of Barrow et al. (8) that state C has V Å 3/2 and as such shows neither L doubling nor hyperfine splitting.

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TABLE 5 Experimental and Calculated Bandheads of the B r X1 2P1 / 2 Bands and Vibrational Intervals DG£/1 / 2 in the B State (cm01 )

209

Bi nucleus, they proposed that D might be identified with the 2P1 / 2 state arising from the configuration sp 4p*s* also exhibiting only one p* electron. (ii) The homogeneous perturbation of X2 ( V Å 3/2) by A1 ( V Å 3/2) affects vibrational levels £ Å 6 and 5 of X2 , which lie 800–1 500 cm01 below the lowest level of A1 (see below). This suggests that the energy shifts observed for the vibrational levels £ Å 4 and 5 of the B state are caused by perturbation of state B ( V Å 1/2) by D ( V Å 1/2). According to the results of Barrow et al. (8), D has the largest re of all known states of BiO. Therefore the relative position and the avoided crossing of the potential curves of states B and D should be similar to that for states X2 and A1 . H ( V Å 1/2)(H } X1 , H r A)

A number of strong red-degraded bands with sharp heads in the R branches are observed in both the FT spectrum between 16 000 and 24 000 cm01 (Figs. 1d–h) and the laser excitation spectrum between 390 and 510 nm (Figs. 2c–f), which are readily assigned to a transition between a new state H and the ground state X1 . The FT spectrum was measured at

D2P1 / 2 , E, F

In the present work, no new information has been obtained on the highest known valence state D and on the two Rydberg states E and F. Concerning knowledge about these states we refer to the original work of Bridge and Howell (4) and Barrow et al. (8) and the constants quoted in Table 3. In the present work state D is of interest for two reasons: (i) Barrow et al. (8) found that in the D R X1 bands, at intermediate J values, where the D levels are not affected by predissociation, the width of the lines is a factor of about 10 smaller than that of lines of comparable J observed in the transitions A2 , B, C } X1 . They concluded that the sharp lines in D R X1 bands arise by cancellation of h.f.s. in states D and X1 . Assuming then that the major cause of the h.f.s. in X1 is magnetic interaction of the p* electron with the

FIG. 13. LIF fluorescence spectra of the £* Å 0, £9 progressions of the B r X1 (a) and B r X2 (b) transitions of BiO. The spectral resolution is É3 nm.

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NIR emission of the H r A2 system selected with a bandpass filter could be used to measure a ‘‘clean’’ LIF excitation spectrum of the H R X1 bands. The £* Å 0, £9 progression of the H r X1 system is observed when Bix vapor is photolysed with 248 nm KrF excimer-laser radiation in the presence of O2 or NO2 (Fig. 24) showing that state H is preferentially populated in the reactions (5a,b). To get more information about the nature of the H state, the chemiluminescence spectrum of the H r X1 transition was measured at spectral resolutions up to 0.05 cm01 . Fig-

TABLE 6 Band Wavenumbers, Relative Intensities, and Transition Probabilities of £* Å 0, £9 Bands of the B r X1 , B r X2 , and B r A2 Transitions of BiO

FIG. 14. LIF fluorescence spectra of the £* Å 0, £9 (a) and £* Å 1, £9 (b) progressions of the B r A2 transition of BiO. The spectral resolution is É3 nm.

a resolution of 0.5 cm01 , and the wavenumbers of 26 bandheads were obtained with an accuracy of approximately 0.2 cm01 . The data are collected in Table 9 together with calculated vibrational separations in state H. The latter data were fitted to Eq. [3] with a standard deviation of 0.34 cm01 . Finally, the bandheads were fitted with fixed vibrational constants of the upper and lower states. The constants deduced from the fits are given in Table 3. Once the energy of state H was known, a band sequence observed in the NIR region near 6200 cm01 (Fig. 21) could be identified to be the D£ Å 0 sequence of the H r A2 transition (Table 9). No other sequences of this transition have been observed showing that the D£ Å 0 bands must have large Franck–Condon factors, i.e., that states H and A2 have similar potential curves and equilibrium internuclear separations re . A fluorescence spectrum of the £* Å 0, £9 progression of the H r X1 transition is readily observed when state H, £* Å 0 is pumped in the 0–0 band (Fig. 22). In the NIR region, fluorescence of the D£ Å 0 and 01 sequences of the X2 r X1 transition and the D£ Å 0 sequence of the H r A2 system are observed in the LIF spectrum (Fig. 23). No emissions of the H r X2 or H r A1 systems have been detected. The Copyright q 1998 by Academic Press

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FIG. 15. Medium-resolution spectrum of the 0–2 band of the B r X1 transition of BiO (a). The resolution is 0.1 cm01 . b: Expanded section of the spectrum showing the width of the line groups.

ures 25a,b show the 6–0 band. Because of the low S/N the exact structure of the band and the number of branches is not recognizable. However, it is evident that the band contains very narrow lines. The minimum line width is É0.06 cm01 , i.e., limited by the resolution of the spectrometer (Fig. 25b). High-resolution (0.05 cm01 ) LIF excitation spectra of the 2–0 band (Fig. 26) revealed an interesting band structure. The band consists of narrrow lines with linewidths of °0.1 cm01 and of lines that are split in 10 h.f.s. components. The width of the line groups is É1 cm01 , i.e., about twice

as large as the h.f.s. splitting in the X1 2P1 / 2 ground state. A closer look at the spectrum of Fig. 25 shows that the 1-cm01 broad groups of h.f.s. lines also show up in the 6–0 band. These findings suggest that H is the V Å 1/2 component of a 2P state in which the h.f.s. splitting is nearly as large as in the X1 2P1 / 2 ground state. Although the bands are rather weak, the D£ Å 0 sequence of the H r A2 transition has been measured at a resolution of 0.02 cm01 . The spectrum clearly shows that in these bands the lines show large h.f.s. (width of the line groups É0.5 cm01 ) confirming the above

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FIG. 16. LIF excitation spectra of BiO measured with Ar carrier gas under conditions of maximum concentration of BiO in the X2 2P3 / 2 state. The total pressure was 1 Torr and the partial pressure of O2 É 50 mTorr. The following dyes, Schott color-glass filters, and Hamamatsu photomultipliers were used, a: Coumarin 2, UG 11 / BG 12, R1104; b: Coumarin 102, UG 11 / BG 12, R1104.

conclusion that the rotational levels of H show h.f.s. comparable in magnitude to that of the X1 ground state levels. I ( V Å 1/2)(I } X1 , I r A2 )

Other series of emission and absorption bands show up in the chemiluminescence and LIF excitation spectra in the range of 18 500–25 000 cm01 (Figs. 1 and 2), which can be assigned to a transition between a second new state I and the ground state X1 . The bands also are degraded to the red

and have sharp heads in the R branches. The wavenumbers of the heads of 25 bands and the deduced vibrational spacings in state I are collected in Table 10. The DG£/1 / 2 data and the bandheads were fitted with standard deviations of 0.12 and 0.15 cm01 , respectively. The results of the analyses are given in Table 3. Figure 27 shows an LIF spectrum measured when level I, £* Å 0 was excited in the 0–0 band. In the photomultiplier range only the £* Å 0, £9 progression of fluorescence bands is observed. In the NIR region the D£ Å 0 and D£ Å 01

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FIG. 17. Medium-resolution LIF excitation spectrum of the 0–0 band of the B R X2 transition of BiO. The spectral resolution is É0.15 cm01 .

sequences of the X2 r X1 system show up together with a weak band at 1285 nm, which is identified to be the 0–0 band of the I r A2 transition (Fig. 28). This band is too weak to be observed in the FT spectrum. A search was made for transitions from I to X2 and A1 , but no emissions were detected. Figure 29 shows a medium-resolution spectrum (0.05 cm01 ) of the 3–0 band of the I r X1 system. Again the number of branches is not obvious. Apparently the bands have no Q branches. This would suggest that state I has V Å 1/2. Because of the V doubling in the ground state, the bands should contain two P and two R branches. As is seen from the spectrum, either two of these branches are very weak or, at the present spectral resolution, are overlapping in pairs over large ranges of J. The width of the lines likely is close to the width of the features shown in Fig. 29b, i.e., É0.2 cm01 . Thus the rotational levels of I must show approximately 60% of the h.f.s. of the X1 ground state. Figure 30 shows a section of a high-resolution LIF excitation spectrum (0.05 cm01 ) of the 2–0 band. The band likely consists of four branches. As in the FT spectra, all lines show a width of approximately 0.2 cm01 . These findings confirm the above assumption that state I has V Å 1/2 and exhibits substantial hyperfine structure. J ( V Å 3/2)(J } X1 , J } X2 , J r A1 )

In the FT spectrum, four red-degraded bands are observed in the wavenumber range 22 500–25 000 cm01 (Figs. 1d,e), which do not belong to any of the transitions discussed before. Because the separations of the bandheads could be measured with an accuracy of É0.2 cm01 it was immediately

obvious that the bands belong to a progression from a common upper level £* to vibrational levels £9 Å 1, 2, 3, and 4 of the X1 ground state. The LIF excitation and fluorescence spectra (Figs. 2b–d and 31) prove that the bands are the £* Å 0, £9 progression of a transition from a third new state J to the ground state. In the LIF spectrum (Fig. 31) also the transitions J, £* Å 0 r X2 , £9 show up with intensities comparable with those of the J r X1 bands. Weak features observed at 712, 740, and 727 nm are identified as the 0– 0 and 0–1 bands of J r A1 and the 0–7 band of J r X2 , respectively. Some weak J r X2 bands also are observed in the FT spectrum (Figs. 1f–h), and state J could be excited from X2 by the laser. The wavenumbers of the heads of the J r X1 and J r X2 bands obtained from the FT spectrum are given in Table 11 together with the vibrational spacings DG£/1 / 2 used to deduce vibrational constants. The results of the analyses are given in Table 3. Table 12 gives the relative intensities of the J r X1 and J r X2 bands and band transition probabilities A£ =£ 0 calculated from the relative intensities and the radiative lifetime of the J state. The ratio of the transition probabilities of the transitions AJ0X1 and AJ0X2 is found to be about 1:3.7. Attempts were made to measure a high-resolution spectrum of the J r X1 bands, but at resolutions of õ0.1 cm01 the bands became very weak and nearly disappeared in the noise. On the other hand, some bands of the J r X2 system, which have only low intensities in the low-resolution spectra, became quite prominent showing simple structures with one R and one P branch only and narrow lines (Figs. 32a,b). These findings suggest that J has V Å 3/2 and that the rotational levels in J exhibit no h.f.s. and V doubling. In

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FIG. 18. LIF fluorescence spectra of transitions from the £* Å 0 level of the C 2D3 / 2 state of BiO. The spectral resolution is É1.5 nm.

that case the J r X2 bands are expected to show the observed simple structure and narrow lines whereas the J r X1 bands will consist of six branches with lines split in 10 h.f.s. components each. G ( V Å 3/2)(G r X2 )

Some weak bands with narrow lines and single R and P branches were observed in high-resolution Fourier transform spectra in the region 8000–12 000 cm01 , which cannot be attributed to transitions between the states hitherto discussed. Preliminary rotational analyses showed four of them to have

a common upper state and the separations of the vibrational levels of the lower state to be identical with those between the £ Å 3, 4, 5, and 6 levels of X2 (Table 2). Therefore, we assign the bands to transitions from a fourth new state G to X2 . The simple structure of the bands (Fig. 33) and the missing of h.f.s. shows that G has V Å 3/2. Because no band series originating from lower vibrational levels of this state could be observed, we assign the common upper state of the four main bands to the £* Å 0 vibrational level of G. Five other bands with narrow lines were observed, which, from their rotational constants, are attributed to the 0–7, 1–

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TABLE 7 Experimental and Calculated Bandheads of the C r X1 2P1 / 2 Bands and Vibrational Intervals DG£/1 / 2 in the C State (cm01 )

radicals in levels X2 , £. Some weak bands showing up in the LIF spectra after pumping of levels H, £ Å 1, 2 (Figs. 34a,b) are tentatively assigned to transitions G, £* Å 1 r X2 , £9 and G, £* Å 2 r X2 , £9. The £ Å 1, 2 states of G likely are populated by near-resonant ( DE ° 300 cm01 ) intramolecular energy transfer from the pumped levels H, £ Å 1, 2. K ( V Å 1/2)(K } X1 , K } X2 , K r A2 )

In the UV range between 350 and 380 nm, a series of strong bands was observed in the LIF excitation spectrum ( Fig. 2b ) , which, from the separation of the bandheads, were identified to be the £ *, £ 9 Å 0 progression of a TABLE 8 Relative Intensities and Transition Probabilities of y* Å 0, y9 Bands of the C r X1 and C r X2 Transitions of BiO

3, 1–6, 1–7, and 1–8 transitions of the G r X2 system. The results of the preliminary rotational analyses of the bands are given in Table 13. Also given in Table 13 are the rotational constants obtained from analysis of the 0–2 band of the J r X2 system (Fig. 32). From the B values of the levels X2 , £ Å 2, 3, and 4, Be Å 0.30519 cm01 and ae Å 0.00242 cm01 are obtained. The rotational constants B0 and B1 calculated with these values of Be and ae and the experimental values of B5 and B6 (Table 13) were used for simulation of the bandshapes of the X2 r X1 bands (Table 1, Figs. 6–9). Attempts were made to excite low vibrational levels of state G via the G R X1 and G R X2 transitions but were unsuccessful. The G R X1 bands obviously are too weak to be observed in the LIF excitation spectrum, and the search for G R X2 bands was disturbed by strong absorption bands of Bi 2 in the wavelength range 700–800 nm showing up under conditions optimized for high concentrations of BiO

a

Calculated by use of the measured lifetime of the C state.

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FIG. 19. LIF fluorescence spectrum of transitions from the £* Å 0 level of the C 2D3 / 2 state of BiO pumped in the 0–4 band of the C R X2 transition. The spectral resolution is É3 nm.

transition from X1 to a fifth new state K . The assignment was confirmed by the fluorescence spectrum measured with the monochromator when the level K , £ * Å 0 was excited in the 0 – 0 band ( Fig. 35 ) . Three progressions of bands show up, which can be assigned to the transitions K r X1 , K r X2 , and K r A2 . The relative intensity distributions of the bands in the £ * Å 0, £ 9 and £ * Å 1, £ 9 progressions are very similar to those in the B , £ * Å 0, 1 r X1 , X2 , £ 9 or J , £ * Å 0, 1 r X1 , X2 , £ 9 progressions. This shows that the potential energy curves of these three electronic states exhibit very similar equilibrium internuclear distances. Once knowing the wavenumbers and the relative intensities of the bands, some weak bands of the K r X1 system were also found in the FT spectrum ( Fig. 1b ) . The wavenumbers of the observed bandheads are given in Table 14 together with the derived DG£/1 / 2 data for state K . The spectroscopic constants obtained from the analysis of the data are given in Table 3. The relative intensities of the £ * Å 0, £ 9 bands of the K r X1 , K r X2 , and K r A2 transitions ( Fig. 35 ) were measured under controlled conditions and used to derive transition probabilities A£ =£ 0 of the bands ( Table 15 ) . The ratio of the transition probabilities of the K r X1 , K r X2 , and K r A2 transitions is found to be 1:0.41:0.18. The low vibrational levels of K could also be excited from the X2 state. Figure 36 shows the excitation spectrum of the 1 – 0 band of the K R X2 transition measured at a resolution of É0.15 cm01 . The band is found to consist of many branches ( probably six ) , and the lines do not show h.f.s. broadening. These findings as well as the intensity ratio of the K r X1 , K r X2 , and K r A2 systems suggest that K has V Å 1 / 2.

A1 ( V Å 3/2)(A1 R X1 ); X2 ( V Å 3/2), y ú 6 (X2 R X1 )

In the red and near-infrared region a number of bands show up in the LIF excitation spectra (Figs. 2f–n), which cannot be assigned to transitions to the A2 state (Table 16). The bands appear under conditions used for studying excitations from the ground state, i.e., when only a few millitorrs of CO2 are used for the production of BiO radicals. This fact as well as the separation of the bands (Table 16) show that they originate from the X1 ground state of BiO. In the range 860–980 nm, no more LIF excitation bands were observed. At medium-spectral resolution (0.2 cm01 ), all the bands show structures similar to those of the A2 R X1 bands with sharp heads in the R branches. The wavenumbers of the bandheads and tentative assignments, which are consistent with the assignment of the observed G r X2 ( £9 Å 3– 8), C r A1 , X2 ( £9 ú 6), and J r A1 , X2 ( £9 ú 6) fluorescence bands are given in Table 17. Among the LIF excitation bands in the range 760–860 nm, one can definitely differentiate two groups of bands excited from the X1 ground state. The upper state for the first group is supposed to be the A1 ( V Å 3/2) state responsible for the perturbation of the levels £* ¢ 5 of the X2 state. The LIF excitation bands ending in low vibrational levels ( £* Å 0, 1, 2) of A1 should be characterized by small isotope shifts at the replacement of 16O2 by 18 O2 . This was qualitatively confirmed by comparison with spectra of Bi 18O. Because of the perturbation, the vibrational levels of A1 and X2 , £ ¢ 5 cannot be well described with the standard expression for anharmonic oscillators. Therefore, calculation of accurate isotope shifts, and hence reliable identification of the bands, was not possible. Estimates of the electronic energy Te and the vibrational constants ve and

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FIG. 20. FT spectrum of the 1–1 band of the C r X2 system. The spectral resolution is 0.05 cm01 .

ve xe were deduced from a fit of the 1–0, 2–0, 2–1, 3–0, and 3–1 bands shown in Table 17. According to these results (Table 3), the A1 state lies 2666 cm01 below A2 and has a somewhat larger vibrational constant ve . The second group of LIF excitation bands in the region 780–860 nm are assigned to transitions X2 , £* ¢ 7 R X1 , £9 Å 0, 1. These transitions with large D£ values should show large isotope shifts when comparing spectra of Bi 16O and Bi 18O. Again this effect was qualitatively ascertained. As X1 and X2 have nearly equal re values, X2 R X1 transitions with large D£ values are expected to have very small

Franck–Condon factors. The fact that such bands are observed thus indicates that the vibronic states X2 , £ Å 6–9 are strongly mixed with the low vibronic states of A1 such that the transitions gain transition probability from the A1 R X1 transition. Only two vibrational levels X2 , £ Å 7 and 9 are found in the excitation spectrum. The levels X2 , £ Å 7 and 8 were observed in the G r X2 chemiluminescence and the C r X2 and J r X2 fluorescence spectra. A number of unidentified bands are found in the region l õ 750 nm, which likely are transitions to perturbed high levels of A1 and X2 (Table 16).

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TABLE 9 Experimental and Calculated Bandheads of the H r X1 2P1 / 2 and H r A2 Bands and Vibrational Intervals DG£/1 / 2 in the H State (cm01 )

a Numbers in parentheses give the o-c differences from the fit with fixed vibrational constants. b Numbers in parentheses give the differences between the experimental data and band wavenumbers calculated with the constants given in Table 3.

Attempts to measure fluorescence spectra of the A1 r X1 or A1 r X2 transitions failed. When low vibrational levels of A1 are pumped, the fluorescence appears mainly in the X2 r X1 bands in the NIR region. This was seen from a comparison of excitation spectra when the fluorescence was measured in the photomultiplier range ( l ° 900 nm) and in the Ge detector range ( l ¢ 1100 nm). In the latter case the A1 R X1 bands are enhanced. The fluorescence observed in the photomultiplier range is mostly emission from state A2 showing that there is also efficient collisional energy transfer from the perturbed levels of A1 and X2 to A2 . When

pumping the unknown band at 655.3 nm with the laser, the spectrum consists mainly of £* Å 2, £9 and £* Å 1, £9 bands of the A2 r X1 system showing that efficient electronic relaxation is populating the near-resonant £* Å 2 level of state A2 . This assumption is confirmed by the buildup of the timeresolved fluorescence intensity observed when pumping in the band at 655.3 nm. Bands of the A1 r X2 transition are expected to lie in the NIR range at wavelength above 1500 nm. Because of the low sensitivity of the infrared detectors ( Ge, InSb ) and strong emissions of the X2 r X1 and A2 r X1 bands,

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FIG. 21. FT spectrum of the D£ Å 0 sequence of the H r A2 transition of BiO. The spectral resolution is 0.5 cm01 .

observation of A1 r X2 bands in this range will be difficult. RADIATIVE LIFETIMES AND RATE CONSTANTS FOR QUENCHING

Time-resolved measurements of the laser-induced fluorescence were performed with the aim of getting first information on the radiative lifetimes of the electronic states of BiO and their stability toward collisional quenching. Knowledge of the lifetimes was hoped to be helpful for identification

of the states. Detailed studies of the rate constants and channels of electronic relaxation in collisions with molecular gases, as recently have been performed for the lower states of PbF (26), were beyond the scope of the present work. A2 , H, I, J, K, B, C

Measurements of the decay times of all states yielding fluorescence in the photomultiplier range were straightforward. Band-pass filters were used to select single bands or band groups in the fluorescence spectra of the

FIG. 22. LIF fluorescence spectrum of the £* Å 0, £9 progression of the H r X1 transition of BiO. The spectral resolution is É3 nm.

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FIG. 23. LIF fluorescence spectrum of BiO in the NIR range measured with the Ge detector when state H, £ Å 0 was pumped in the 0–0 band of the H R X1 system. The spectral resolution is É10 nm.

A2 , H , I , J , K , B , C r X1 systems, and the decay curves were obtained with good signal-to-noise ratio by averaging the signals from 500 or 1000 laser shots in the digital oscilloscope. The decay curves were transferred to a laboratory computer and analyzed by use of a least-squares fitting program. All measured decay curves were single exponentials. First the effective lifetimes of the states were measured as a function of the pressure of Ar carrier gas. A minimum of about 1.2 Torr Ar carrier gas from

both the Ar / Bix and the Ar / O 2 ( CO2 ) /MW inlets with a few millitorrs partial pressure of O2 or CO2 was necessary to generate a sufficient concentration of BiO radicals in the gas flow. For all states, plots of the reciprocal decay times vs. Ar pressure could be fitted by straight-lines. The radiative lifetimes obtained from the intercepts and the quenching rate constants deduced from the slopes of the lines are given in Table 18. Similar results were obtained for the £ Å 1 levels and when the measurements

FIG. 24. Emission spectrum of the H r X1 system of BiO observed when Bix vapor was photolyzed with 248 nm KrF excimer laser radiation in the presence of O2 . The spectral resolution is É1.5 nm.

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FIG. 25. FT emission spectrum of the 6–0 band of the H r X1 system of BiO at a resolution of 0.05 cm01 .

were performed with He carrier gas. In a second step, the pressure of Ar carrier gas was kept constant at about 1.5 Torr, and O2 or CO2 was added as quenching gas. The quenching rate constants are collected in Table 18. For all states, He and Ar show similar quenching rate constants ranging from 3 1 10 013 ( I , £ Å 0 ) to 1.6 1 10 011 cm3 s 01 ( B , £ Å 0 ) . Quenching by O 2 and CO2 is more efficient, but the results show that in the measurements with rare gases the decay times are not influenced by the few millitorrs of O2 or CO2 needed for the production of BiO. The good reproducibility of the data indicates that quenching by the traces of Bix vapor and bismuth com-

pounds in the gas flow likewise can be neglected. In all states the rate constants for deactivation of the £ Å 1 levels are somewhat larger than those for the £ Å 0 states suggesting that vibrational relaxation plays a role and occurs with rate constants similar to those for quenching. At higher pressures of the carrier gas, vibrational relaxation also shows up in the LIF spectra of the £ * Å 1 levels by the appearence of £ * Å 0, £ 9 bands. The error limits of the radiative lifetimes and of the rate constants for collisional deactivation were estimated from the standard deviations of the corresponding parameters of the fits, the estimated errors in measurements of partial pressures,

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FIG. 26. High-resolution LIF excitation spectra of sections of the 2–0 band of the H R X1 transition of BiO. The resolution is É0.05 cm01 .

the S / N in the measurements of single decay curves, and the possible influence of the small partial pressures of O2 or CO2 used in the experiments. A1 ( V Å 3/2)

As has been noted above, no emissions that definitely could be assigned to A1 r X1 or A1 r X2 bands have been detected in the LIF and FT spectra. Probably, there are two reasons for this: (i) the population of A1 is depleted by efficient collisional exchange of excitation energy between states A1 , £ ¢ 4 and A2 and between states A1 , £ ° 3 and

X2 (see below); (ii) the transition probabilities of both, the perpendicular A1 r X1 transition and the parallel A1 r X2 transition in the NIR range, probably are much lower than that of the parallel A2 r X1 transition. The latter assumption would lead to a rather long radiative lifetime of A1 . G ( V Å 3/2)

The only time-resolved emissions possibly originating from level G are those of the weak bands observed following laser pumping of level H, £ Å 1 and 2 (Figs. 34a,b). The emissions showed a complicated time behavior with a

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TABLE 10 Experimental and Calculated Bandheads of the I r X1 2P1 / 2 Bands and Vibrational Intervals DG£/1 / 2 in the I State (cm01 )

FIG. 27. LIF fluorescence spectrum of the £* Å 0, £9 progression of the I r X1 transition of BiO. The spectral resolution is É3 nm.

was found to be about 400 ms. However, systematic measurements of the reciprocal decay times as a function of Ar or He pressure did not show straight-lines. When the emission of the D£ Å 0 sequence was measured with the monochromator at narrow slit-widths, it could be observed that with increasing pressure of carrier gas or added gas the maximum of the fluorescence is shifting from a low pressure value of 1425 nm to a high pressure value of 1411 nm. This shift approximately corresponds to the wavelength separation of the 6–6 and 0–0 bands of the X2 r X1 system. The observation shows that, at low pressure, mostly levels £* Å 6 and 5 of the X2 state are populated in the relaxation process suggesting that the relaxation proceeds via the A1 state and is strongly influenced by the perturbation between states A1 and X2 . This relaxation pathway clearly deserves more detailed studies, which were beyond the scope of the present work. Lifetime measurements by excimer-laser photolysis also a Numbers in parentheses give o-c differences from the fit with fixed vibrational constants in units of the last digit. b Not used in the fit.

buildup and effective decay times in the range of 10–15 ms. No reliable information on the radiative lifetime of state G could be deduced from these measurements. X2 2P3 / 2

X2 r X1 fluorescence could be generated by laser pumping of higher-lying states of BiO as well as by excimer-laser photolysis of different parent compounds. Quantitative studies of the radiative lifetime and of rate constants for quenching of the X2 state, however, turned out to be difficult. In the LIF work, good signal intensities were obtained when the D£ Å 0 bands of the X2 r X1 system were selected by a band-pass filter. Under most conditions, the decay time

FIG. 28. LIF fluorescence spectrum of BiO in the NIR range measured with the Ge detector when state I, £ Å 0 was pumped in the 0–0 band of the I R X1 system. The spectral resolution is É10 nm.

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FIG. 29. FT emission spectrum of the 3–0 band of the I r X1 system of BiO at a spectral resolution of 0.05 cm01 .

met some problems. The fluorescence signals from 248 nm photolysis of Bi 2O3 and BiONO2 were too weak for systematic time-resolved measurements with the fast Ge detector. Under optimum conditions (laser pulse energy, vapor pressure of parent compound), the decay times were found to be independent of the nature and pressure (3–5 Torr) of He or Ar carrier gas and were in the range 400–450 ms. Stronger signals were obtained from 248 nm photolysis of Bix vapor in the presence of O2 or NO2 . However, in this case the measurements were disturbed by superposition of the BiO fluorescence by a broad structured spectrum, which we attribute to emissions of Bi 2 . The same spectrum shows up in

the photolysis of Bix vapor without addition of O2 or NO2 . It possibly consists of transitions from high vibrational levels of the recently discovered low-lying a11 state of Bi 2 (33). The Bi 2 molecules may be excited by photolysis of Bi 4 clusters or by recombination of Bi atoms. The spectral overlap of the BiO and Bi 2 emissions results in double exponential decay curves with long tails. The effect was less when NO2 was used instead of O2 and when a few Torrs of N2 quenching gas were added to the system. Figure 37 shows the time behavior of the X2 r X1 emission at different NO2 concentrations. The curves clearly show a buildup supporting the assumption that the excited BiO molecules are pro-

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FIG. 30. High-resolution LIF excitation spectrum of a section of the 2–0 band of the I R X1 system of BiO. The resolution is É0.05 cm01 .

duced by a fast reaction of excited bismuth atoms or molecules. At higher NO2 pressure both the buildup and the decay time are faster due to higher rates of formation and quenching of the X2 state. Figure 38 shows a decay curve measured from photolysis of a Bix /NO2 /N2 mixture at low NO2 partial pressure. The decay time is 470 ms in good agreement with the previous data. From the bulk of all measurements we conclude that the lifetime of the X2 state is 480 { 100 ms. Figure 39 shows Stern–Volmer plots for quenching of the X2 state by O2 , NO, and NO2 . All other gases tested were found to show much lower quenching efficiencies. The re-

sults of the quenching measurements are collected in Table 18. From the fact that O2 is much more efficient than N2 and CO, it can be concluded that quenching by O2 proceeds via the E–E energy exchange process [6], BiO* (X2 , £ Å 0) / O2 r BiO (X1 , £ Å 0) 1 01 / O* 2 (a Dg , £ Å 0) 0 796 cm

[6]

Using the principle of detailed balancing the rate constant for the reverse process can be calculated by

FIG. 31. LIF fluorescence spectrum of the £* Å 0, £9 progressions of the J r X1 and J r X2 transitions of BiO. The spectral resolution is É3 nm.

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TABLE 11 Experimental and Calculated Bandheads of the J r X1 2P1 / 2 and J r X2 2P3 / 2 Bands and Vibrational Intervals DG£/1 / 2 in the J State (cm01 )

DISCUSSION

Nature of the Electronic States and Transitions

The assignment of the experimentally observed electronic levels to spin–orbit states arising from the different electron configurations of BiO is greatly facilitated by the theoretical work of Alekseyev et al. (28). The ground state and the low-lying excited states of BiO arise from the following electron configurations (12, 25, 28) (I) . . . ( s ) 2 ( s*) 2 ( s ) 2 ( p ) 4 ( p*) (II) . . . ( s ) 2 ( s*) 2 ( s ) 2 ( p ) 3 ( p*)2 (III) . . . ( s ) 2 ( s*) 2 ( s ) 1 ( p ) 4 ( p*)2 (IV) . . . ( s ) 2 ( s*) 2 ( s ) 2 ( p ) 4 ( s*)

X 2P r Pr , 2Pi , 2P, 2Fi , 4Pi 2 / 2 0 2 S , S , D , 4S 0 2 / S 2

The relative change of the equilibrium internuclear distances and the vibrational constants by promotion of an electron from the bonding s and p orbitals to the antibonding p* TABLE 12 Relative Intensities and Transition Probabilities of £* Å 0, £9 Bands of the J r X1 and J r X2 Transitions of BiO

a Numbers in parentheses give the o-c differences from the fits with fixed vibrational constants in units of the last digit. b Not used in the fit.

k06 /k6 Å

Q(BiO(X2 ))Q(O2 (X )) exp( 0 DE/kT ) [7] Q(BiO(X1 ))Q(O2 (a))

where Q(i) are the partition functions of the molecules in the electronic states involved and DE is the reaction energy. Because both molecules have very similar rotational and vibrational constants in their ground states and first excited states, the translational, rotational, and vibrational parts of the partition functions cancel out and Eq. [7] reduces to

k06 /k6 Å

Qel (BiO(X2 ))Qel (O2 (X )) exp( 0 DE/kT ) Qel (BiO(X1 ))Qel (O2 (a))

[8]

Using Qel (BiO(X1 )) Å 2, Qel (BiO(X2 )) Å 2, Qel (O2 (X )) Å 3, Qel (O2 (a)) Å 2, DE Å 0796 cm01 , T Å 298 K, and the value of k6 given in Table 17, k06 is calculated to be 1.8 1 10 010 cm3 s 01 .

a Calculated by use of the measured lifetime of the J state. b Estimated value, band overlapped with stray-light.

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FIG. 32. High-resolution FT spectrum of the 0–2 band of the J r X2 transition of BiO. The spectral resolution is 0.025 cm01 .

orbital has been discussed by Rai and Rai (12). A substantial increase in re and a decrease in ve as compared with the ground state is expected for the states arising from configurations II and III. This assumption has been confirmed by the recent theoretical calculations (28). According to the theoretical work, p* r s* excitation also leads to weakening of the BiO bond such that the 2S / state arising from IV is expected to have similar ve and re values to those of the states arising from configurations II and III. As is seen from Table 3, all excited states up to C have similar ve values of

approximately 500 cm01 . The Franck–Condon pattern of transitions between the excited states (H r A2 (Fig. 21), I r A2 (Fig. 28), K r A2 (Fig. 35), B r A2 (Figs. 14a,b)) show that the states likewise have similar re values. These findings suggest that the excited states A1 , A2 , G, H, I, J, K, B, and C all arise from the configurations II, III, and IV. Identification of the nature of the states and transitions is based on the following facts and assumptions: (i) The ground state of BiO is the 2Pr state arising from

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FIG. 33. High-resolution FT spectrum of the 0–4 band of the G r X2 transition of BiO. The spectral resolution is 0.025 cm01 .

configuration I with spin–orbit components X1 2P1 / 2 and X2 2P3 / 2 . The vibrational constants of states X1 and X2 and the electronic energy of X2 are well established. (ii) In X1 2P1 / 2 , the rotational levels show large L doubling (p É 0.187 cm01 ) and large magnetic hyperfine splitting. The width of the groups of h.f.s. levels is about 0.52 cm01 and nearly independent of J and of the L-doublet component. All transitions to or from the X1 ground state thus should show strong hyperfine splitting of the rotational lines and a doubling of branches unless, by chance, the h.f.s. and the L doubling in the upper state are similar such that the effects in both states cancel.

(iii) No observable L doubling and h.f.s. occur in the X2 2P3 / 2 and other V Å 3/2 states (31). (iv) Angular momentum coupling in BiO is close to Hund’s case (c). The states and transitions are essentially characterized by the quantum numbers V. Transitions with DV Å 0 and DL Å 0 should show P and R branches only, in transitions with DV Å 0 and DL Å {1 (e.g., 2S1 / 2 r 2 P1 / 2 ) weak Q branches may show up. Transitions with DV Å {1 should consist of P, Q, and R branches of regular relative intensities. Because of the DS Å 0 selection rule in the limit of Hund’s case (a) coupling, in systems with DL Å 0 (e.g., 2P r 2P ), the parallel subtransitions with DV

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TABLE 13 Spectroscopic Constants Deduced from Rotational Analyses of G r X2 and J r X2 Bands (cm01 )

Å 0 should show higher intensities than the perpendicular transitions with DV Å {1. (v) In transitions between the ground state levels X1 , X2 and excited states arising from the electron configurations II, III, and IV because of the large changes of re , long progressions of bands are expected with maximum intensity in bands with D£ Å 4–8. On the other hand, in transitions between any two of the excited states only bands with low values of D£ should show up.

Taking this as a starting point, the experimental results have allowed classification of most transitions by DV and thus identification of the V values of all observed states. In Fig. 40, the experimental Te values are assigned to low-lying spin–orbit states of BiO calculated by Alekseyev et al. (28). All experimental states can readily be correlated to theoretical levels lying in the same relative order but systematically about 1000–1500 cm01 lower than the experimental data. The assignments are supported by the good agreement of experimental and theoretical ve values (28) and radiative lifetimes. The literature data for the ground state X1 2P1 / 2 are mostly confirmed by the present results. The vibrational constants of X1 now are based on DG£/1 / 2 values derived from six transitions. A rough analysis of the high-re-

solution spectrum of the 0 – 0 band of the X2 r X1 transition ( Figs. 10a,b ) confirmed the large L doubling and yielded an improved value for the hyperfine splitting constant dX 1 . Since there is no other low-lying state with a vibrational constant as large as that of the 2Pr state from configuration I, assignment of X2 to the V Å 3 / 2 component of this state is obvious. The electronic energy of X2 and the vibrational spacings DG£/1 / 2 up to £ Å 6 have been deduced from the X2 r X1 , J r X2 and G r X2 transitions. Observation of narrow lines and single R and P branches in the J r X2 and G r X2 bands ( Figs. 32 and 33 ) confirms that there is neither observable L doubling nor h.f.s. splitting in X2 . The structures of the bands and band sequences of the X 2 r X1 transition are well understood. Levels £ ¢ 6 of X2 are strongly perturbed, their energies are shifted down, the £ * Å 6, £ 9 bands show modified shapes and enhanced intensities. These experimental findings are well explained by the theoretical calculations ( 28 ) of avoided crossing of the potential curves of X2 and of the V Å 3 / 2 component of the 4Pi state denoted A1 . Efficient collisional coupling of A1 and X2 in the region of perturbation is a major pathway in the collisional relaxation of A1 and all higher-lying states. Preferential population of

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FIG. 34. LIF fluorescence spectra of the £* Å 1, £9 (a) and £* Å 2, £9 (b) bands of the G r X2 transition of BiO at a resolution of É3 nm with pumping of the 1–0 (a) and 2–0 (b) bands of H R X1 .

levels X2 , £ Å 6, 5 clearly shows up after laser pumping of low vibrational levels of A2 . This relaxation channel explains the hot vibrational population of X 2 observed in the steady-state chemiluminescence system with Ar carrier gas, which is unusual for a metastable state with a radiative lifetime of 480 ms. Most likely, avoided crossing of the analogous lowest V Å 3 / 2 potential curves will explain the similar perturbations observed in the X2 r X1 bands of BiS and BiSe as well as the failure to observe the X2 r X1 transition of BiTe ( 30 ) .

The spectroscopic information on state A1 still is meagre. Its electronic energy and vibrational constants are not well determined. Identification of A1 as a V Å 3 / 2 state is based on the perturbations in the vibrational structure of X2 discussed above and on observation of some bands with sharp lines and single R and P branches in the 8000 – 10 000 cm 01 region, which are assigned to G r A1 transitions. As is shown in the theoretical work of Alekseyev et al. ( 28 ) , the first excited state of BiO, which was originally assigned as A 2P, actually is dominated by

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FIG. 35. LIF fluorescence spectrum of the £* Å 0, £9 progressions of the K r X1 , K r X2 , and K r A2 transitions of BiO. The spectral resolution is É3 nm.

the 4P l – s state. Therefore we assign A1 to the V Å 3 / 2 component of the 4Pi state arising from configuration II. The V Å 1 / 2 component of this state is the well-known A state, which, for more clarity, is denoted A 2 4P1 / 2 ( 28 ) . The electronic energy and the vibrational constants of A 2 are well ascertained. In our high-resolution spectra of A2 r X1 bands we find no hints to Q branches. Comparison of the h.f.s. pattern in the A2 r X1 and X2 r X1 bands indicates that the rotational levels of A 2 show h.f.s. splitting. Both results confirm that A2 has V Å 1 / 2. TABLE 14 Experimental and Calculated Band Origins of the K ( V Å 1/2) R X1 2P1 / 2 LIF Excitation Bands and Vibrational Intervals DG£/1 / 2 in the K State (cm01 )

a

Numbers in parentheses give the obs.-calc. differences from the fit with fixed vibrational constants in units of the last digit.

The simple structure of the G r X2 bands (Fig. 33) shows that state G exhibits no V doubling and h.f.s. splitting and, therefore, most likely has V Å 3/2. The vibrational assignments of the upper state, and hence the T 0 value of G, still need confirmation by further measurements. In line with the relative order of states found in the theoretical work (28), G is assigned to the V Å 3/2 component of the first excited 2 P state (Fig. 40). This assignment is supported by the observed relative strength of the G r X2 and G r X1 transitions. The electronic energies and the vibrational constants of the other new states H, I, J, and K are validated by observation of different transitions involving the states and measurements of the band systems with the FT spectrometer as well as with the LIF technique. In the H r X1 and I r X1 bands, the linewidths definitively are less than 0.52 cm01 expected from the h.f.s. in the ground state (Figs. 25, 26, 29, and 30). Consequently, the rotational levels of H and I likewise must show h.f.s., which is partly cancelling or enlarging the effect of h.f.s. in the ground state. These findings suggest that H and I have V Å 1/2 and, together with the observed relative intensities of the transitions, suggest assignment of the states to the V Å 1/2 components of the 2P and 4S 0 states as shown in Fig. 40. In the work of Alekseyev et al. (28) it is shown that the two V Å 1/2 states are heavily mixed over a large range of r values. As a consequence, state H is pushed down in energy such that the doublet splitting of the first excited 2P state is reduced to about 300 cm01 . From comparison with lighter group V oxides (19), this splitting was expected to be 1500–2000 cm01 . According to theoretical work of Kopp and Hougen (34), for a 4S 10/ 2 state far from

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TABLE 15 Relative Intensities and Transition Probabilities of £* Å 0, £9 Bands of the K r X1 , K r X2 , and K r A2 Transitions of BiO

The simple structure of the J r X2 bands (Fig. 32) and the narrow width of the lines show that J has V Å 3/2. The high intensity of the perpendicular J ( V Å 3/2) r X1 ( V Å 1/2) transition indicates that J is not a component of a 2P state and thus supports identification of J with the V Å 3/ 2 component of the 4S 0 state as suggested by the theoretical calculations (28). Assignment of K to a V Å 1/2 state and its identification as the 2S 1// 2 state calculated to lie in this energy region ( 28) is inferred from the observed large number of branches in the K R X2 bands (Fig. 36) and the relative intensities of the K r X1 , K r X2 , and K r A2 systems. The present results confirm the electronic energies and the vibrational assignments in states B and C given by Bridge and Howell ( 4 ) and Barrow et al. ( 8 ) . Assignment of B and C to the calculated 2S 01 / 2 and 2D3 / 2 states ( Fig. 40 ) again is based on the observed band structures and the relative intensities of the fluorescence systems. The bands of the B r X1 system likely consist of four branches ( Fig. 15 ) . Hyperfine Splitting

other interacting states, the V-doubling constant p should be approximately four times greater than the rotational constant B resulting in branches with unusual spacings, and the relative branch intensities in 1/2–1/2 transitions are determined by the ratio of two transition moments m\ and m⊥ allowing a great variety of intensity patterns. However, because of the strong spin–orbit mixing, the expressions derived in this work likely are not applicable to BiO. High-resolution studies of the H r X1 , I r X1 , and B r X1 transitions are needed to derive reliable rotational constants of the states and to confirm the assignments.

BiO was among the first molecules for which effects of magnetic hyperfine splitting were observed in electronic spectra ( 6, 8 ) . The experimental results of Barrow et al. ( 8 ) prompted Atkins ( 13 ) to investigate the magnetic hyperfine interaction in a Hund’s case ( c ) coupling scheme. Applying his results to BiO, he concluded that the observed J-independent h.f.s. splitting should show up only in 2P1 / 2 states arising from electron configurations with an unpaired p* electron centered at the 209 Bi nucleus, i.e., in the X1 2P1 / 2 ground state arising from the s 2p 4p* and in the high-lying D 2P1 / 2 state from the sp 4p*s* configuration. All other states with paired p* electrons were assumed to show no first-order hyperfine interaction. The results of the present work show this assumption to be wrong. Hyperfine splitting amounting to Ç100% and Ç60% of the splitting in the ground state is also found in the V Å 1 / 2 components of the first excited 2P and 4S 0 states H and I . In all states, the splitting is found to be nearly independent of J , showing that it is caused by electron spin – nuclear spin interaction ( hyperfine doubling ) . In the Hund’s case ( a ) limit, the interaction energy DW Å 0 for the 2P3 / 2 state ( 31 ) , and for the 2P1 / 2 state it is given by DW Å |

d(J / 1/2) 4J(J / 1) 1 [F(F / 1) 0 I(I / 1) 0 J(J / 1)].

For 2P states, the upper sign applies to the e levels and the lower to the f levels. Figure 41 shows an energy level

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FIG. 36. Medium-resolution LIF excitation spectrum of the 1–0 band of the K R X2 transition of BiO. The resolution is É0.15 cm01 .

diagram and the lines expected in a transition between two 2P1 / 2 states with h.f.s. splitting parameters d of the same sign and magnitude. One can see that for the Pee , Pf f , Ree , and Rf f lines the effect of h.f.s. splitting is cancelling such that all h.f.s. components coincide in a narrow line. On the other hand, for the Qef and Qfe lines the splittings in both states add up and the width of the groups of h.f.s. lines is twice the splitting in either of the states. This is exactly what is found in the H 2P1 / 2 } X1 2P1 / 2

bands ( Figs. 25, 26 ) and supports the identification of states G and H to be components of an inverted 2P state. The high intensities of the Q branch lines in the H ( V Å 1 / 2 ) r X1 ( V Å 1 / 2 ) bands ( Fig. 26 ) likely is caused by the strong mixing of H with the V Å 1 / 2 components of the 4S 0 and of 2S states ( 28 ) . The medium-resolution spectra of the I 4S 01 / 2 } X1 2P1 / 2 bands ( Figs. 29, 30 ) show that the bands consist of at least four branches and state I exhibits h.f.s. splitting

FIG. 37. Time decay of the X2 r X1 emission of BiO ( D£ Å 0 bands) with photolysis if Bix vapor in the presence of 3 Torr N2 and 5 mTorr (a), 10 mTorr (b), 30 mTorr (c), and 100 mTorr (d) of NO2 .

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TABLE 16 Unassigned and Tentatively Assigned Bands of Bi 16O Found in the LIF Excitation Spectra

73

tational levels of A2 must exhibit 20% of the h.f.s. splitting of X1 , which is partially cancelling the 0.52 cm01 broad splitting observed in the X2 r X1 bands. As was shown in the theoretical work ( 28 ) , A2 is mixing with a number of other V Å 1 / 2 states and, at larger r values, contains 10 – 20% admixture of X1 2P1 / 2 . Therefore, h.f.s. splitting of about the observed magnitude and with the same sign of the splitting constant d is to be expected in this state. According to the present results, the states B and K assigned to 2S 0 and 2S / do not show h.f.s. splitting ( Figs. 15, 17, and 36 ) though they have V Å 1 / 2 and arise from a similar electron configuration with two p* electrons. This leaves us with the question why states H and I show large hyperfine structure splitting though they have no unpaired p* electron centered at the Bi nucleus. Radiative Lifetimes, Transition Probabilities, and Electronic Quenching

In Table 18, the experimentally determined radiative lifetimes of the £ Å 0 levels of the states X2 , A2 , G, H, I, J,

TABLE 17 Excitation, Fluorescence, and Chemiluminescence Bands Assigned to Have A1 , y ° 5, or X2 , y ¢ 7 as Upper or Lower Levels

amounting to about 60% of that of the X1 state. The 0.2cm01 wide line groups showing partial cancellation of h.f.s. splitting in the upper and lower states then are P and R lines. There is some evidence ( Fig. 30 ) that the bands contain weak Q branches with large h.f.s. splitting like the H } X1 bands. Thus the rotational levels of I likely show the same type of hyperfine structure like those of state H with a 40% smaller splitting constant d . In fact, because of the strong mixing of states H and I ( 28 ) this is to be expected. Measurements at higher spectral resolution and S / N are needed to clarify this point. In the A 2 2P1 / 2 r X1 2P1 / 2 bands, all lines show nearly the same h.f.s. splitting ( Fig. 11 ) , but the width of the line groups ( 0.42 cm01 ) is about 20% less than in the X2 2P3 / 2 r X1 2P1 / 2 bands ( Fig. 10 ) , which reflect the h.f.s. splitting in the X1 ground state. Therefore the roCopyright q 1998 by Academic Press

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TABLE 18 Radiative Lifetimes and Rate Constants for Quenching of Electronic States of BiO

K, B, and C are compared with the theoretical results of Alekseyev et al. (28). The agreement is reasonably good, except for X2 , where the discrepancy is more than a factor of 5. From measurements of relative band intensities, the ratios of transition probabilities for the subtransitions to the two ground state components X1 and X2 were determined for states C, B, J, and K (Figs. 13, 18, 31, and 35 and Tables 6, 8, 12, and 15). Comparison of these results with the data calculated from the theoretical partial lifetimes t1 and t2 reveals some bigger discrepancies. Although the ratio AB0X1 / AB0X2 is predicted to within 25%, the calculated ratio AK0X1 / AK0X2 is by a factor of 30 larger than the experimental result. The calculated ratio AJ0X1 /AJ0X2 Å 0.10 likewise is by at least a factor of 3 too small (Fig. 31). The discrepancies reflect the difficulty of calculating transition probabilities if the states involved are mixtures of several l –s states, and

the mixing strongly changes with internuclear distance (28). Errors of the mixing coefficients enter the calculated transition probabilities in square. The overall uncertainty of the theoretical result is expected to be largest for transitions that are forbidden in the Hund’s case (a) limit and can acquire transition moment by spin–orbit mixing only, i.e., intercombination and spin–orbit transitions. This explains the large discrepancy found for the lifetime of the X2 state. The very high intensity of the X2 r X1 transition in the Bix /O * 2 system clearly favors the shorter experimental lifetime of X2 (Table 18). The relative intensities of the D£ Å /1 bands of the X2 r X1 system (Fig. 5) cannot be explained by the vibrational populations or by Franck–Condon factors. The increase of the transition probabilities from the 1–0 to the 5–4 bands rather must be caused by a strong increase of the electronic transition moment in this sequence. This reveals

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FIG. 38. Time decay of the straylight and of the X2 r X1 emission of BiO ( D£ Å 0 bands) with photolysis if Bix vapor in the presence of 3 Torr N2 and °0.5 mTorr of NO2 .

a more general feature of the electronic states and transitions in heavy-atom containing molecules: although for allowed transitions of light molecules the electronic transition moment often can be assumed to be constant over the relevant range of internuclear distance, and hence the radiative lifetime is constant for different vibrational levels and the band transition probabilities are proportional to the Franck–Condon factors, in heavy-atom containing molecules these approximations likely are more the exception than the rule.

Because of the strong spin–orbit mixing of the electronic states and the often encoutered changes of the spin–orbit mixing with internuclear distance the electronic transition moment will often show a strong dependence on internuclear distance resulting in strong changes of the electronic part of the band transition probabilities within a progression and different radiative lifetimes of the vibrational levels. In general, the experimental and theoretical results support the identification and assignment of the new states. State B

FIG. 39. Plots of the reciprocal effective decay time of the X2 r X1 emission of BiO ( D£ Å 0 bands) as a function of O2 , NO, and NO2 pressure.

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has the shortest radiative lifetime of all states studied. Forbidden quartet–doublet intercombination transitions acquire transition probability only by spin–orbit mixing and stealing transition moment from strong allowed transitions. Therefore, the very short lifetime of B contradicts the identification of B to be the 4S 0 state arising from configuration III given by Barrow et al. (8). Except for the X2 state, the rate constants for quenching of the excited states by He and Ar are on the order of 10 012 cm3 s 01 , and the efficiencies for O2 and CO2 mostly are higher by 1 – 2 orders of magnitude. This suggests that quenching occurs by intramolecular electronic relaxation via the ladder of states down to X2 . Quenching of X2 by He, Ar, and most molecular gases is very inefficient allowing the buildup of high concentrations of X2 in the chemiluminescence, photolysis, and LIF systems. Quenching of the X2 state by O2 is several orders of magnitude more efficient than by comparable molecular gases like N2 , CO, and CO2 . This strongly indicates that quenching by O2 proceeds by the near-resonant energy transfer process [ 6 ] , i.e., the endothermic reverse reaction of the energy exchange process, which is supposed to be the key step in the excitation mechanism of BiO in the Bix / O 2* chemiluminescence system. The estimated rate constant of the reverse process of 1.8 1 10 010 cm3 s 01 shows the energy transfer reaction from O 2* ( a ) to BiO to be the most efficient gas phase reaction of singlet oxygen molecules hitherto known ( 35 ) . A similar high efficiency for near-resonant spin – orbit excitation by O 2* ( a ) has been observed for PbF ( 26 ) . Excitation Processes

The first and key step in the mechanism populating excited states of BiO in the Bix / O 2* chemiluminescence system obviously is the exothermic E – E energy exchange reaction [ 06 ] . Because of its long radiative lifetime and collisional stability, high concentrations of BiO in the metastable state X2 can build up and energy pooling processes in Eq. [10 ] BiO*(X2 ) / O * 2 (a) r BiO*(A1 , A2 , rrr) / O2 (X ) [10] become feasible. Further excitation of BiO to states up to 32 000 cm01 may proceed by other energy pooling processes of O * 2 and BiO *. Important intermediates in this mechanism possibly are the experimentally unknown metastable A 3 4P5 / 2 and A4 4P01 / 2 components of the 4P state near 15 500 cm01 and the 2F5 / 2 and 2F7 / 2 states near 24 000 cm01 ( Fig. 40 ) . The fact that addition of oxygen atoms has no influence on the chemiluminescence intensities indicates that chemical reactions are of minor importance in the excitation mechanism. The photolytic and chemical processes producing ex-

FIG. 40. Experimental and calculated energies of the electronic states of BiO.

cited BiO radicals on photolysis of Bix vapor in the presence of O2 or NO2 deserve further attention. The preferential population of the H 2P1 / 2 state ( Fig. 24 ) shows that the reaction is exothermic by at least 2.5 eV and supports the assumption that electronically excited atoms or molecules are involved ( Eqs. [ 5a ] and [ b ] ) . SUMMARY AND CONCLUSIONS

The present chemiluminescence and LIF studies have yielded a wealth of new information on the electronic states and spectra of BiO. Four previously known and and 14 new electronic transitions have been studied. Six new states have been detected and together with five previously known states were characterized by their electronic energies, vibrational constants, radiative lifetimes, and stabilities toward quenching. The detailed ab initio calculations of electronic energies, vibrational constants, and transition probabilities by Alekseyev et al. ( 28 ) have allowed safe assignment of the previously known and the new electronic states to spin – orbit states arising from the lowest electron configurations. Large hyperfine structure splitting has been observed in two of the new states, which cannot be explained by the previously used

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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. FIG. 41. Schematic view of the rotational energy levels and hfs splitting in two 2P1 / 2 states with similar h.f.s. parameters d. The diagram illustrates the cancelling of hfs splitting in the P and R lines and the addition of the effect for the Q lines.

22. 23. 24. 25.

model ( 13 ) . Several states have been found ( J , K , B , C ) to combine with both components X 1 and X2 of the 2 Pr ground state, and relative band transition probabilities have been measured for the corresponding subtransitions. The data will allow LIF measurements of relative populations in the X1 ground and the metastable X2 state of BiO molecules generated in photolysis processes and chemical reactions.

26. 27. 28. 29. 30. 31. 32.

ACKNOWLEDGMENTS 33. Financial support of this work by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged. The authors are grateful to J. M. Brown, Oxford, for helpful discussions.

34. 35.

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