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New spectroscopic observations in electric discharges through carbon monoxide
Chemical Physics 276 (2002) 167–179 www.elsevier.com/locate/chemphys
New spectroscopic observations in electric discharges through carbon monoxide Daniel Cossart Laboratoire de Photophysique Mol eculaire du CNRS and Institut de Physico-Chimie, Mol eculaire, B^ atiment 213, Universit e de Paris-Sud, 91405 Orsay Cedex, France Received 27 July 2001; in final form 25 October 2001
Abstract Two types of electric discharges were used: (a) In a supersonic expansion of pure CO gas. (b) In a U-shaped tube immersed in liquid nitrogen where either pure CO or mixtures of CO þ He, CO þ Ne, CO þ NO are introduced. In both cases, the analyzed emissions were exclusively those issuing from the negative glows of the discharges. Fourier Transform spectra were recorded in the 28 000–17 000 cm1 visible spectral region and also in the 8000–2000 cm1 infra-red interval. Comparison of the two sets of spectra showed that spectra (a) correspond to excitation of the primary species whereas spectra (b) result from excitation of final compounds produced in the positive column. In the latter case, the main new observations are: (i) Selective excitation of particular upper state vibrational levels in the E3 R –a3 P Herman system. (ii) Appearance of the C1 Rþ –B1 Rþ ð0; 0Þ Rydberg–Rydberg infrared band whereas the associated optical C1 Rþ –A1 P Herzberg transition is absent. (iii) ‘‘Anomalous’’ vibrational distribution in the ground state vibration–rotation spectra. All the above observations are concomitant with previously reported IR emissions involving very high rotational levels (up to J ¼ 120) in the ground state. To explain them, processes involving dissociative recombinations of dimer cations are suggested. Ó 2002 Published by Elsevier Science B.V.
1. Introduction Among the remaining spectroscopic problems of interest concerning carbon monoxide, one relates to the CO2þ doubly charged ion, and another to the quintet states of neutral CO. As only vibrational analyses were available on these subjects [1,2], I attempted to obtain the corresponding spectra in the gas phase. With this double objec-
E-mail address: [email protected] (D. Cossart).
tive, two special emission sources were developed, both working at low temperature in order to make easier identification of new bands in spectral regions where the known emissions of CO are very congested. The two sources, a Penning-type electric discharge in a supersonic expansion, and a Negative glow emission in a U-shaped tube immersed in liquid nitrogen, are described in [3] and [4], respectively. The supersonic jet source allowed detection of several emission lines that could belong to the CO2þ di-cation, whereas a number of newly
0301-0104/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 3 0 1 - 0 1 0 4 ( 0 1 ) 0 0 5 7 3 - 0
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observed bands remained unidentified [5]. As for the U-shape tube, it was initially built with the objective of observing the gas phase spectrum of the intersystem a005 P–a03 Rþ and a005 P–d3 D transitions of CO. Bands of the latter have been earlier detected in the 350–550 nm spectral region in a CO/Ar matrix [2] and have been tentatively assigned to the above transitions on the basis of ab initio calculation [6] on the lowest quintet states of CO. Attempts to observe these bands in the gas phase proved negative, and several reasons to justify this failure will be given here later. However, by means of the cooled U-shape tube, we observed in the Dv ¼ 1 sequence of the ground state vibration–rotation spectrum (2500–1800 cm1 ) extremely high rotational excitations (up to J ¼ 120) that likely result from þ dissociative recombination of the ðCOÞ2 dimer cation [4]. The latter emissions appeared concomitant with ‘‘anomalous’’ vibrational distributions in the ground state together with selective state level excitation, particularly the v0 ¼ 7, 11 vibrational levels of the E3 R state, and v0 ¼ 0 level of the Rydberg C1 Rþ state. Thorough examination and comparison of the spectra obtained with the two emission sources was carried out, providing assignments of the previously unidentified bands, as well as possible mechanisms to explain qualitatively the above observations.
2. Experimental The Penning-type electric discharge in a Supersonic Expansion and the negative glow emission in a U-shaped tube immersed in liquid nitrogen are hereafter labelled, respectively, SE and UST experiments. A Brucker IFS 120 HR Fourier Transform spectrometer was equipped with a Hamamatsu 1P 21 photomultiplier to record the spectra in the 28 000–17 000 cm1 visible region, and with a cooled InSb detector for the 8000–1800 cm1 IR spectral interval. About 200 interferograms were co-added in each experiment with resolutions of, respectively, 0.2 and 0:01 cm1 for the visible and IR spectral regions.
2.1. SE experiment The experimental conditions are given in [3], i.e. pumping with a 3700 l/s diffusion pump backed by a 50 m3 =h mechanical pump. Before expansion, pure CO gas was introduced at a pressure of 1 bar, whereas a pressure of about 103 mbar was measured in the ionization cell. The image of the spectrometer entrance diaphragm projected onto the expansion region was a circle (diameter d ¼ 1 cm) contiguous to the nozzle throat. In that case, only emissions during a time interval shorter than d=u, u being the speed of the isentropically expanding gas, can be observable. Assuming that h þ u2 =2 ¼ h0 , where h0 and h are the gas enthalpies per unit mass at the expansion source and at a given point in the Mach bottle, respectively, the basic kinetic theory of gases provides the following expression of the gas velocity 1=2 [7]: u ¼ ½k 2cðT0 T Þ=m ðc 1Þ , where k is the Boltzmann constant, m the molecular weight, (T0 T ) the cooling effect, and c ¼ Cp =Cv the heat capacity ratio. The latter can be roughly estimated (c ¼ 1:4) from the relation: ð3=2Þ ðCp –Cv Þ= Cv ¼ (translational degrees of freedom)/(rotational + translational degrees of freedom) ¼ 3/5 in the case of a diatomic molecule. By setting m ¼ 28, c ¼ 1:4 and T0 T ¼ 270 K, one obtains: u ¼ 750 m/s. It means that radiative transitions whose upper state lifetimes exceed d=u ¼ 13 ls cannot be detected in the SE experiments. They include rotation–vibration and optical intersystem transitions whose mean durations are often in the millisecond range. 2.2. UST experiment In contrast to the preceding situation where only rapid initial processes can be observed, the spectra recorded when using the discharge tube immersed in liquid nitrogen described in [4] correspond to emissions from final products. Indeed, before to entering the observation region (negative glow) the gases, initially introduced in the anode side at a pressure of about 4 Torr, went through a 60 cm length U-shaped positive column, the pumping way being in the cathode side. Experiments were carried out successively with pure CO,
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CO þ He, CO þ Ne, and finally CO þ NO. In each of the three last cases, 50% flowing mixtures were used. The pressures in the cathode region were only several tenths of a Torr (except in the CO þ He case) since the major part of the gases was trapped in the positive column cooled at liquid nitrogen temperature. It should be noted that, in the UST experiments with pure CO or CO plus rare gases, the tube walls rapidly become blackened due to solid carbon deposition, whilst such a carbon deposition did not occur in the CO þ NO case, but condensation of N2 O produced in the discharge gave rise to a blue–violet coloration of the tube.
3. Results Fig. 1 reproduces parts of the emission spectra between 26 300 and 21 125 cm1 . Spectra a and b correspond, respectively, to the UST and SE emission sources with pure CO gas. The visible-toIR spectral observations (30 000–2000 cm1 ) are summarized in Table 1 showing the differences when using either the SE or the UST emission sources. Aside from those resulting from the dif-
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ferent rotational temperatures, the main differences are the following: 3.1. Triplet transitions Bands of the d–a Triplet system involving high v0 levels (v0 P 10) appear in both the SE and UST spectra, while those of the a0 –a Asundi system appear only in the SE spectrum where the low rotational temperature strongly enhances the intensity of the first J-lines in the a03 Rþ –a3 P2 subbands. Sets of 25 and 49 such bands were observed in the SE spectra of the a0 –a and d–a transitions, respectively. Band head measurements for several of them have already been reported in the Krupenie review [8] but, as no rotational analysis was performed until now for these high v0 triplet transition bands, I give in Tables 2 and 3 the high resolution measurements for the strongest line in each sub-band. They may help the identification of CO in low temperature spectra. Fig. 2 shows, as an example, the Triplet d–a ð10; 1Þ band recorded by means of both the SE (spectrum b) and UST (spectrum a) sources with the strongest R lines indicated. Note that, while the R13 (2) and R2 (1) lines are obviously the most
Fig. 1. Fourier Transform emission spectra between 21 000 and 26 500 cm1 of CO excited in the UST source (spectrum a) and SE source (spectrum b). See text for the meanings of the SE and UST initials. Note in the UST spectrum the strong e–a Herman bands involving the v0 ¼ 7 and 11 levels, and the absence of the C–A Herzberg bands. Bands of COþ labelled CT and BJ correspond, respectively, to the Comet–Tail and Baldet–Johnson systems.
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Table 1 Summary of the spectral emissions of CO observed under various experimental conditions
Vibration Rotation Asundi Triplet Herman ngstr€ A om Herzberg Rydb–Rydb Rydb–Rydb
1
þ
XR X1 Rþ a03 Rþ –a3 P2 d3 D–a3 P e3 R –a3 P B1 Rþ –A1 P C1 Rþ –A1 P C1 Rþ –B1 Rþ E1 P–B1 Rþ
SE pure CO
UST pure CO
UST CO þ He
UST CO þ Ne
UST CO þ NO
n.o. n.o. v0 up to 20 v0 up to 25 v0 up to 12 o o n.o. n.o.
vmax ¼ 2 and 8 vh J.o v0 up to 10 v0 up to 11 v0 ¼ 7 and 11 o n.o. o n.o.
vmax ¼ 6 vh J n.o. n.i. n.i. n.i. n.i. n.i. n.i. n.i.
vmax ¼ 2 and 8 vh J.o n.i. v0 up to11 v0 ¼ 7 and 11 o n.o. o n.o.
vmax ¼ 6 and 32 vh J.o n.i. n.i. n.i. n.i. n.i. n.i. n.i.
SE: Penning-type excitation in supersonic expansion; UST: Negative glow in a U-shaped tube cooled at liquid nitrogen temperature; vh J: very high J values (up to J ¼ 120); o/n.o.: observed/ not observed. n.i.: not investigated; The most significant observations are indicated by bold face figures.
Table 2 High v0 sub-bands observed for the Triplet system of CO v00 v0
0
1
2
10
22696.29 770.46 23653.72 728.04 24592.70 667.13 25513.18 b
20981.95 1056.25 21939.39 2013.52 22878.34 952.60 23798.88 873.06 24700.96 81.64 25584.45 659.44 26449.34 524.21 27295.46 370.30 28122.57 95.65
19296.83 370.67 20254.23 328.38 21193.15 267.38 22113.73 87.77 23015.79 91.73 23899.38 974.01 24764.30 839.02 25610.38 b 26437.45 514.05 27245.27 319.66
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
3
20457.81 531.76 21359.89 435.73 22243.51 318.01 23108.37 82.84 23954.50 4029.03 24781.63 b b 25663.59 26377.53 451.42 27145.63 219.01 27893.06 965.48
4
b 21556.49 22327.81 402.57 23154.96 231.32 b 24037.26 24750.94 825.13 25518.96 b 26266.40 339.10 26992.55 b 27696.63 b
5
20725.95 99.99
22365.32 –
23923.23 – 24669.01 b 25391.70 464.95 26099.21 – 26780.25 –
Figures in cm1 correspond to wavenumber measurements of the R13 (2) and R2 (1) lines in, respectively, the 3 D3 –3 P2 and 3 D2 –3 P1 sub-bands (see text); b: blended by stronger emissions of CO or COþ .
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Table 3 High v0 sub-bands observed for the Asundi system of CO v00 v0
0
1
10 11 12 13 14 15 16 17 18 19 20 21
18002.45
16288.08 17297.03 18287.54 b 20211.80 21146.44
2
16602.41 17573.81 18525.92 19461.31 20377.34
3
16870.04 17805.36 18721.43 19619.30 20498.00
4
5
6
17094.78 17992.90 18871.31 19731.74 20579.88
18135.78 18976.00
16558.82 18229.14
Figures in cm1 correspond to wave number measurements of the Q13 ð2Þ lines in the a0 3 Rþ –a3 P2 sub-bands (see text); b : blended by stronger emissions of CO or COþ .
Fig. 2. Fourier Transform emission spectra of the d3 D–a3 P ð10; 1Þ transition band of CO excited in: (a) UST source, (b) SE source (see text).
prominent in the d3 D3 –a3 P2 and d3 D2 –a3 P1 subbands, the rotational intensity distribution is more spread out in the third d3 D1 –a3 P0 component because of the different relative line strength ratios [9]. For this reason, band head measurements are given in Table 2 only for the d3 D2 –a3 P1 and d3 D3 –a3 P2 components. It can be remarked that, because of the low rotational temperature, the intensity distributions in the ð10; 1Þ sub-bands
shown in Fig. 2 are very different from those displayed by Carroll [10] for the ð3; 0Þ band recorded at room temperature. One can obtain a rough estimation of the rotational temperatures in spectra a and b of Fig. 2 by measuring the ratio between the R13 (2) and R13 (3) line intensities in the d3 D3 –a3 P2 sub-bands. These particular peaks, the most intense in a given band, were chosen not only for this reason but, above all, because they
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represent the ‘‘quasi unperturbed’’ intensities, since the other possible way to radiatively depopulate the upper d3 D3 sub-levels, i.e the d3 D3 –X1 Rþ transition, implies violation of two selection rules. On the other hand, the spin-uncoupling is minimum for these first J -levels. This situation is confirmed by the scarcity of the corresponding observed subbands. Only one vibrational d3 D3 sub-level (v0 ¼ 4) has been indeed observed in the d–X spectra [14]. The above mentioned intensity ratio is equal to: s ¼ I½R13 ð2Þ=I½R13 ð3Þ / SJR0 ¼3 exp½F 0 ð3Þ hc=kT =SJR0 ¼4 exp F 0 ð4Þ hc=kT ¼ SJR0 ¼3 =SJR0 ¼4 exp F 0 ð4Þ F 0 ð3Þ hc=kT ; where 0 0 0 SR J 0 ¼ ðJ þ 2ÞðJ þ 1Þ=4J
is the R-branch line strength coefficient for a 3 D–3 P transition [9] and F 0 ð4Þ–F 0 ð3Þ the energy difference between the corresponding 3 D3 upper-state levels which is equal to m½R13 ð3Þ m½Q13 ð3Þ ¼ 14:52 cm1 . The s ratios were determined to be 1.17 and 1.64 for, respectively, the spectra a and b of Fig. 2 leading to the corresponding rotational temperatures in the UST and SE experiments: T 77 and 34 K. Note that the rotational tem-
perature in the UST experiment is exactly that of the liquid nitrogen bath, so that the d3 D levels are surely populated by vertical excitation of thermalized CO molecules in their ground state. Fig. 3 shows, as an example, the ð16; 3Þ band of the Asundi a0 –a system recorded when using the SE source. Note that the v0 P 10 levels were not observed with the UST source. It should be emphasized that only the a03 Rþ –a3 P2 component was detectable for this electronic transition, which is the reason why in previous work the 25 bands listed in Table 3 remained unidentified [5]. However, examination of the calculated low J line strengths for this electronic transition [9] fully explain this apparently striking observation. Similarly to the Triplet system considered above, Table 3 gives the measurements for the Q13 (2) lines. Knowledge of the high v0 level rovibronic constants for the a03 Rþ ; d3 D and e3 R states results chiefly from analysis of the vacuum UV absorption spectra [14–16], all the considered triplet-singlet transitions being spin forbidden. In that case, their intensities are borrowed essentially through spin– orbit interaction from the allowed 1 P, 1 Rþ –1 Rþ singlet–singlet transitions. An extended deperturbation treatment has been performed by Field et al. [17] providing experimental values for the perturbation matrix elements together with deperturbed rovibronic constants. Local coincidences between
Fig. 3. Fourier Transform emission spectrum of the a0 3 Rþ –a3 P2 ð16; 3Þ transition sub-band of CO excited in the UST source (see text). Note that the two associated sub-bands were not detectable.
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triplet and singlet levels cause, in this case, irregular band intensity variations from one vibrational level to the next in the upper state. Tables 2 and 3 show, on the contrary, regular trends in the development of the Condon parabolae for the d–a and a0 –a transitions. In order to test the hypothesis that the CO triplet–triplet emissions in the SE experiment are due to vertical electron impact excitation from the v ¼ 0 ground state of the monomolecular CO, the band intensity ratios Iv0 ;v00 =Iv0 þ1;v00 þ1 in a given sequence of the d–a transition are compared in Table 4 with the corresponding Franck–Condon factor ratios: 0 2 hvd jvX ¼ 0ihv0d jv00a i=hv0d þ 1jvX ¼ 0ihv0d þ 1jv00a þ 1i ; where the Franck–Condon factors are obtained with a Morse potential for the ground state and RKR potentials for the a3 P and d3 D states [18]. Rather than normalize all the band intensities to
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that of a reference band, I preferred to consider successive Iv0 ;v00 =Iv0 þ1;v00 þ1 intensity ratios in several sequences in order to avoid as much as possible the sensitivity variation of the detector and transmittance of the filters used in different spectral intervals. Similarly to the rotational temperatures determination in the spectra of Fig. 2, the experimental intensities were estimated from measurements of the lowest-J peak maxima in the d3 D3 –a3 P2 sub-bands. Comparison of the experimental and calculated values given in Table 4, shows that the hypothesis of vertical electron impact excitation from the CO (v ¼ 0) ground state is the most likely. In this respect, a mistake must be corrected in [5] where it has been argued that the a0 , d, e–a transitions are induced by radiative recombination of the COþ ions. This hypothesis was proposed to justify the enhancement of the neutral CO emissions
Table 4 Observed and calculated band intensity ratios in the CO, d3 D a3 P sequences Band (v0 ; v00 )
Iv0 ;v00 (arb. units)
½ðIv0 ;v00 Þ=ðIv0 þ1;v00 þ1 Þ observed
102 hvd jva i2 ¼ FC1
102 hvd jvX¼0 i2 ¼ FC2
104 ðFC1 FC2 Þ
½ðIv0 ;v00 Þ=ðIv0 þ1;v00 þ1 Þ calculated
ð10; 0Þ ð11; 1Þ ð12; 2Þ ð13; 3Þ
315 634 612 188
0.49 1.04 3.25
2.80 6.16 5.48 1.73
6.85 7.38 7.59 7.52
19.18 45.46 41.59 13.01
0.42 1.09 3.20
ð11; 0Þ ð12; 1Þ ð13; 2Þ ð14; 3Þ
193 511 564 272
0.38 0.91 2.07
1.75 4.84 5.68 3.07
7.38 7.59 7.52 7.17
12.92 36.74 42.71 22.01
0.35 0.86 1.94
ð12; 0Þ ð13; 1Þ ð14; 2Þ ð15; 3Þ
102 367 503 341
0.28 0.73 1.48
1.09 3.60 5.26 4.00
7.59 7.52 7.17 6.69
8.27 27.07 37.71 26.76
0.30 0.72 1.41
ð13; 0Þ ð14; 1Þ ð15; 2Þ ð16; 3Þ ð17; 4Þ
64 210 367 339 153
0.30 0.57 1.08 2.22
0.67 2.56 4.49 4.38 2.17
7.52 7.17 6.69 6.11 5.42
5.03 18.35 30.04 26.76 11.76
0.27 0.61 1.12 2.27
ð15; 1Þ ð16; 2Þ ð17; 3Þ ð18; 4Þ ð19; 5Þ
125 251 280 169 42
0.50 0.90 1.66 4.02
1.78 3.64 4.28 2.91 0.78
6.69 6.11 5.42 4.78 4.19
11.91 22.06 23.2 13.91 3.22
0.54 0.95 1.67 4.32
Note: The Iv0 ;v00 intensities, in arbitrary units, correspond to the peak maxima for the R13 (2) lines; The Franck–Condon factors FC1 and FC2 were calculated with RKR potentials.
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relative to those of the COþ and CO2þ ions when the pumping speed in the SE experiment was reduced. To explain this observation, it seems now more reasonable to suggest that, instead of favouring the neutral CO emissions, reduction of the pumping speed, i.e. pressure increase in the ionization cell, can diminish the ion densities through dissociative recombination, a process very much more efficient than radiative recombination [19]. Such a comparison between the observed and calculated intensities for the d3 D2 –a3 P1 component (the experimental intensities were those of the R2 (1) peaks) shows almost the same agreement, indicating that the d3 D2 sub-levels, like the associated d3 D3 components, relax mainly to the a3 P manifold. The situation appeared quite different for the third d3 D1 –a3 P0 component as well as for the a03 Rþ , e3 R –a3 P bands since many spin–orbit interactions involving the X ¼ 1 quantum number may occur with neighbouring A1 P state levels. While the hypothesis of vertical excitations from the monomolecular CO ground state seems now well established (at least for the d3 D levels) in the case of the SE experiment, the situation appeared different from the UST experiments. Concerning the d–a emission bands, the vibrational intensity distribution remains broad. On the other hand, it has been shown above that the d3 D levels are in thermal equilibrium with the ‘‘bath’’ (T 77 K) so that the excitation process should be the same as in the SE experiment. However, similar to the high v0 , a03 Rþ –a3 P bands which do not appear in the UST spectrum, most of the e3 R –a3 P Herman bands are also absent, except several involving the v0 ¼ 7 and 11 levels. The latter appear with high intensities, as can be seen in Fig. 1 where the ð7; 0Þ, ð11; 0Þ and ð11; 1Þ bands are indicated. In spite of their complexity, rotational temperatures in the latter transitions (T 190 K) were roughly estimated as before by determination of the J values (Jmax 7–8) for the Q3 lines of maximum intensity in the e3 R –a3 P0 sub-bands. It should also be remarked that, under the same experimental conditions, several ‘‘High Pressure’’ Swan bands of C2 are visible, i.e. those involving selective excitation of the d3 Pg , v0 ¼ 6 level. The ð6; 5Þ band is shown in Fig. 1. Note that, while the C2 ‘‘High pressure’’ and CO v0 ¼ 7, 9 Herman
bands appear simultaneously, their intensities are not correlated in different experiments. Finally, it should be remembered that the present experimental conditions are those corresponding to the excitation of very high rotational levels (up to J ¼ 120) in the vibration–rotation spectrum of the CO ground state [4]. 3.2. Singlet transitions ngstr€ om and All the expected B1 Rþ –A1 P A C1 Rþ –A1 P Herzberg bands appear in the SE spectrum of Fig. 1, but the C1 Rþ –A1 P bands with v0 ¼ 0, v00 ¼ 1–4 are completely absent in the UST spectrum. However, under the same experimental conditions (UST, pure CO and UST, CO þ Ne), the Rydberg–Rydberg C1 Rþ –B1 Rþ ð0; 0Þ transition strongly appeared at 5002:885 cm1 . Fig. 4 shows the corresponding spectra obtained by means of the UST source with pure CO and CO + Helium. The rotational temperatures for a 1 þ 1 þ R – R transition can be easily estimated from the Jmax values as T 310 and 110 K for the pure CO and CO þ He experiments, respectively. This suggests that the C1 Rþ levels are initially populated by a dissociation process inducing a rotational temperature higher than that of the ‘‘bath’’, the addition of helium generating a partial thermalization. This band was first observed by Amiot et al. [11] and Roncin et al. [12] together with the E1 P–B1 Rþ ð0; 0Þ band of another Rydberg–Rydberg transition. Note that the latter transition was not observed here. The ratio of the m3 factors (about 100) between the visible C1 Rþ –A1 P and infrared C1 Rþ –B1 Rþ transitions would suggest, at first sight, that the electronic transition moments and Franck– Condon factors are much more favourable for the C–B than for the C–A transition. This hypothesis should be rejected if one considers that, whereas the C–A Herzberg bands (upper state lifetime ¼ 1.5 ns [13]) are clearly present in Spectrum b of Fig. 1, the C–B ð0; 0Þ transition band does not appear in the SE spectra where transitions whose time constant is less than 13 ls can be observed. Moreover, the rather high rotational temperature determined above (T 310 K) is to be contrasted with that (T 77 K) in the d3 D–a3 P optical spectrum.
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Fig. 4. Fourier Transform emission spectrum of the C1 Rþ –B1 Rþ ð0; 0Þ Rydberg–Rydberg transition of CO recorded with the UST source (see text) with and without addition of helium.
3.3. Vibrational intensity distribution in the vibration–rotation spectra As mentioned above, the IR vibration–rotation spectra are not detectable with the SE source since the upper level lifetimes always exceed the 13 ls limit resulting from the high gas expansion speed. The spectra shown in Fig. 5 were obtained when using the UST source. The Dv ¼ 2 sequence was considered since the InSb detector, which is sensitive down to only 2000 cm1 , would limit the observation of an extended Dv ¼ 1 sequence. The lower spectrum a corresponds to excitation of CO + Helium. Emissions from vibrational levels up to v0 ¼ 17 are visible with a maximum intensity around v0 ¼ 6. The intermediate spectrum b obtained with pure CO (or with CO þ Ne) shows two maxima for v0 ¼ 2 and 8. The number of excited vlevels is approximately the same as in spectrum a, but one observes that this is not the case for the upper spectrum c (CO þ NO) where vibrational levels were excited up to v0 ¼ 38. A first intensity maximum at v0 ¼ 6 is similar to that observed for the CO þ He case, whereas a second one appeared at v0 ¼ 32. It should be pointed out again that these ‘‘anomalous’’ vibrational intensity distributions were observed under the same experimental
conditions (pure CO or CO þ Ne, CO þ NO, but not CO þ HE where ro-vibrational distributions were thermalized by collisions with helium atoms) as those producing excitation of very high rotational levels in the Dv ¼ 1 sequence [4].
4. Discussion The above differences between the UST and SE spectra, the latter being taken as a reference of direct excitation of the CO monomer, strongly sugþ gest that dissociative recombinations of the ðCOÞ2 dimer ion play an important role in the UST experiments. In the UST source, the gases were suddenly introduced through a small orifice into the negative glow after passing through a positive column cooled at liquid nitrogen temperature. From a study of the mobility of COþ , COþ 2 and þ ðCOÞ2 in carbon monoxide glow discharges, þ Saporoschenko [20] showed that ðCOÞ2 is the principal carrier ion at small E=p0 values, where E is the electric field and p0 the gas pressure reduced to the corresponding value for 0 °C. When E=p0 increases, the ðCOÞþ 2 density decreases while those of COþ and COþ increase in a correlated manner. The 2 small E=p0 condition is particularly well satisfied in
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Fig. 5. Fourier Transform emission spectra of the Dv ¼ 2 sequence in the ground state of CO excited in the UST source (see text): (a) CO + Helium, (b) pure CO, (c) CO + NO.
the positive column of the UST source, due to the low voltage gradient and the high gas equivalent pressure, the temperature being near to that of the CO condensation. Moreover, it has been shown [20] that, under these conditions, the pressure deþ pendence of the ðCOÞ2 ion intensity is characteristic of a two-body collision: COþ þ CO ! þ ½ðCOÞ2 ; v in which v refers to vibrationally excited levels, the maximum initial vibrational energy being equal to the dissociation energy [21] of the dimer ion D0 ffi IPðCOÞ IP ½ðCOÞ2 ¼ 14:01 ð12:24 0:15Þ eV ¼ 14 270 1210 cm1 . This situation is schematically illustrated in Fig. 6 where values of the dissociation and ionization energies [21] of the neutral dimer are indicated. Possible dissociation limits are also given. Those for which selective and concomitant excitations were observed are shown with heavier traces, i.e: (a) The v 6 6 levels in the X1 Rþ ground state. (b) The v ¼ 7 and 11 of the e3 R state in the Herman system. (c) The v ¼ 0 level of the C1 Rþ Rydberg state in the C1 Rþ –B1 Rþ transition.
With the present data, it is not possible to establish correlations between the dimer and the fragment levels shown in Fig. 6 which appear in a concomitant manner, nor to determine whether the dissociations are direct or indirect through intermediate Rydberg states. Only the following qualitative remarks can be made concerning the four possible dissociation pathways pointed out in Fig. 6: (i) The very high rotational temperatures (Tr ffi 25 000 K) reported in a previous paper [4] for the CO ground state fragments appear only in the v0 6 6 levels. The greatest intensities for these high J-emissions appear in the ð4; 3Þ and ð5; 4Þ vibration–rotation bands [4], while the energy of the highest vibrational level observed (v0 ¼ 6) is Ev¼6 ¼ 12 460 cm1 i.e. just lower than the D0 disþ sociation energy of the ðCOÞ2 dimer ion. Since the higher levels should be predissociated, this observation was pointed out in [4] to support the hypothesis that the ‘‘High rotational temperature þ spectra’’ originate from dissociation of the ðCOÞ2 ion, another argument being that only dissociative
D. Cossart / Chemical Physics 276 (2002) 167–179
177
Fig. 6. Schematic representation of the ground state potential energies of ðCOÞ2 and ðCOÞþ 2 along the C–C internuclear distance with possible dissociation limits. For the latter, heavier traces indicate levels for which selective excitations were observed.
recombination of an ion in a bent electronic state can give rise to the very high rotational energies observed. On the other hand, the mean vibrational energy of the X1 Rþ , CO fragment was determined [4] as: hEv i ¼ 7296 cm1 . When considering the vibrational intensity distribution in Fig. 5(b), one observes that a maximum of re-absorption appears between the v0 ¼ 3–5 levels, whose average energy can be compared with the above mean energy hEv i. The latter can thus preexist in the CO fragments þ before the COþ þ CO ! ½ðCOÞ2 ; v associative collision, which can, in turn, provide the additional contribution to further excite these CO fragments in vibrational levels up to the v0 ¼ 6 level, the higher levels being predissociated along the C–C coordinate. Fig. 7 shows how this additional vibrational energy of the CO fragments can originate from the energy stored in the C–C stretching mode depending on the H bending angle. Nevertheless, due to the number of unknown parameters, it appeared impossible to determine a precise value for the Er =Ev ratio, and consequently for the H angle 2 ðtan2 H ¼ ðpr =pv Þ ¼ Er =Ev Þ. This is unfortunate, since an experimental value would be useful, in conjunction with the ab initio calculated ground þ state potential surface of ðCOÞ2 performed by Blair et al. [22], to estimate the r(C–C) distance for which the dissociation occurs. From energetics considerations, it has been suggested [4] that, in this case, a CO ðX1 Rþ Þþ COða3 PÞ repulsive potential surface should cross that of the dimer ion. However, though the calculation was limited to internuclear , it can be seen in Fig. 2 distances rðC–CÞ 6 2:25 A
(a)
(b)
(c) Fig. 7. Schematic representation of the ðCOÞþ 2 dimer ion showing possible C–C to C–O stretching energy transfers. (a) Near to the equilibrium position. Hcalc eq ffi 144° is the theoretical equilibrium bending angle [22]. (b, c) At high r(C–C) distances (linear geometry) in the turning points positions of: (b) the symmetric and (c) the antisymmetric stretching modes of C–O.
of the paper by Blair et al. [22], where the potential energy curves are drawn in the (r(C–C), H) coordinates, that the O–C–C–O structure approaches . linearity when r(C–C) increases beyond 2.25 A The only unambiguous conclusion that can be drawn in this particular case is that the H angle is significantly different from 180°, because of the very high rotational energy observed, so that the crossing region should be in the vicinity of the potential surface minimum of the dimer ion. We will see below that the situation should be different for the other selectively excited fragment levels observed.
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(ii) The vibrational energy of the e3 R , v ¼ 7 level is Ev¼7 ¼ 7260 cm1 , i.e almost equal to half the available vibrational energy ðD0 ffi 14 270 cm1 Þ. However, the intensity of the associated Herman bands is very much higher than that of the IR emissions considered above. Note also that the rotational temperature in the Herman bands under study, roughly estimated around Tr ¼ 190 K, is much lower than that of the CO(X) fragments determined in [4]: Tr ffi 25 000 K. A different dissociation pathway should thus be considered. The e3 R , v ¼ 11 level possess a vibrational energy Ev¼11 ¼ 10 992 cm1 , i.e about one and half times that of the v ¼ 7 level. Since the intensity and rotational temperature in the corresponding Herman bands do not differ from those in the v ¼ 7 level, one can assume that both the v ¼ 7 and 11 levels are populated through the same dissociative pathway. Although a simple means cannot be found to justify quantitatively the 3/2 vibrational energy ratio for the two e3 R observed levels, one could nevertheless explain why the vibrational energies can be different in two concomitant CO fragments. When considering that the rotational temperatures in cases (i) and (ii) are different by two orders of magnitude, and also that such a difference does not correspond to the energy difference between the COða3 PÞ þ COðX1 Rþ Þ and COðe3 R Þ þ COðX1 Rþ Þ dissociation limits, it seems reasonable to assume that the dissociating ðCOÞþ 2 ion is, in this case, quasi-linear so that the COðe3 R Þ þ COðX1 Rþ Þ potential surface crosses that of the dimer at much larger r(C–C) distances, i.e. near to the dissociation along the C–C coordinate. Figs. 7(b) and (c) are schematic represenþ tations of a linear ðCOÞ2 ion at the time when it breaks. The probability for this event is indeed maximum when the a; s C–O vibrations, considered as local modes, are in their turning positions. Continuous arrows represent the forces applied to the O and C atoms for a given turning point situation, while broken arrows represent the repulsive forces suddenly applied to the two carbon atoms. It can be easily seen that, in the symmetric case (Figs. 7(b), the vibrational energy stored in the C–C bound is equally transferred to the fragments, whereas in the antisymmetric case (Figs.
7(c)) this energy may be unequally partitioned. If one considers, indeed, a ‘‘normal’’ potential curve in the [U, r(C–O)] coordinates, it is reasonable to imagine that several quanta of vibration are more easily accepted in the repulsive than in the attractive region, because of the different slopes in the potential curve and, consequently, of the different vibrational function overlaps. Finally, one could consider an analoguous dissociative recombination of the C3 Oþ ion to explain the appearance of the ‘‘High pressure’’ Swan bands of C2 shown in Fig. 1. However, more information on the ground state potential surface and the dissociation energy of C3 Oþ is required. (iii) The last selectively excited level observed here is the C1 Rþ ð3prÞ, v ¼ 0 Rydberg level. As the rotational temperature (T 310 K) does not correspond to those in the selectively excited e3 R –a3 P Herman bands (T 190 K) or in the vibration–rotation spectra (T 25 000 K), one can suppose that another dissociative pathway should be considered, assuming that this state level is also populated through dissociative recombination of the dimer ion. The small energy difference between this level and the potential energy minimum of the þ ðCOÞ2 ground state imposes a flat dissociative pathway. On the other hand, the high level density of dissociative surfaces which can cross that of the ground state ion is hardly compatible with a selective excitation. An indirect dissociative recombination seems the most likely since symmetry selection rules have to be applied between high Rydberg and dissociative valence states. The purpose here is not to give a theoretical model for explaining this selectivity (calculations on the high Rydberg states and their dissociation limits are needed ), but to show that this selectivity indeed exists, in such a way that neighbouring states like the B1 Rþ ð3srÞ or E1 Pð3ppÞ are not involved in this process. In this way, a population inversion between the B and C states may be established so that an amplified spontaneous emission could induce a collective de-excitation accompanied by a line narrowing of the emission lines [23]. Line width measurements will be carried out in the future on the C–B ð0; 0Þ transition using the highest resolution of our Fourier Transform spectrometer.
D. Cossart / Chemical Physics 276 (2002) 167–179
179
5. Conclusion
References
The spectra obtained with a Penning-type electric discharge in a supersonic expansion of pure CO were taken as reference spectra for the CO monomer, since it was shown that they result from direct vertical electron impact excitation. In this case, the vibrational and rotational intensity distributions show regular variations in agreement with the calculated Franck–Condon factors. In contrast, the spectra obtained from the negative glow of a U-shape tube immersed in liquid nitrogen showed several selective excitations of particular vibronic state levels that likely originate from dissociative recombinations of the ðCOÞþ 2 dimer ion through different pathways that remain to elucidate. Finally, I would like to conclude by making a comment on the possible observation of the a005 P state of CO for which this work was initially begun. The a005 P–a03 Rþ and a005 P–d3 D transition bands that Bahrdt et al. [2] thought to have observed in an argon matrix should necessarily borrow their intensities from triplet–triplet transitions unlike what was believed by Bauschlicher et al. [6] who calculated Franck–Condon factors for the above transitions as if they were allowed. In fact, under the low temperature conditions necessary to detect weak emissions in a crowded spectrum, emissions from the a005 P state are limited to those arising from the 5 P3 component, since this state is inverted [24] with a spin–orbit constant of about 30 cm1 . In that case, only mixing with 3 D3 levels could give intensity to the forbidden quintet–triplet transitions, but these perturbations should be observable in d–a transitions originating in the 3 D3 sub-states with vibrational quantum numbers higher than v ¼ 25, i.e. in a spectral region not investigated here.
[1] M. Lundqvist, P. Baltzer, D. Edwardsson, L. Karlsson, B. Wannberg, Phys. Rev. Lett. 75 (1995) 1058, and references therein. [2] J. Bahrdt, H. Nahme, N. Schwentner, Chem. Phys. 144 (1990) 273. [3] D. Cossart, C. Cossart-Magos, Chem. Phys. Lett. 250 (1996) 128. [4] C. Cossart-Magos, D. Cossart, J. Chem. Phys. 112 (2000). [5] D. Cossart, J.-M. Robbe, Chem. Phys. Lett. 311 (1999) 248. [6] C.W. Bauschlicher Jr., S.R. Langhoff, H. Partridge, J. Chem. Phys. 98 (1993) 8785. [7] R. Campargue, These d’Etat, Paris, 1979. [8] P.H. Krupenie, Nat. Stand. Ref. Data Ser. US Nat. Bur. Stand. 5 (NSRDS-NBS) (1966). [9] I. Kovacs, Rotational Structure in the Spectra of Diatomic Molecules, Adam Hilger, London, 1969. [10] P.K. Carroll, J. Chem. Phys. 36 (1962) 2861. [11] C. Amiot, J.-Y. Roncin, J. Verges, J. Phys. B 19 (1986) L19. [12] J.-Y. Roncin, A. Ross, E. Boursey, J. Mol. Spectrosc. 162 (1993) 353. [13] K.P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979. [14] S.G. Tilford, J.D. Simmons, J. Phys. Chem. Ref. Data. 1 (1972) 147. [15] G. Herzberg, T.J. Hugo, S.G. Tilford, J.D. Simmons, Can. J. Phys. 48 (1970) 3004. [16] J.D. Simmons, S.G. Tilford, J. Res. Nat. Bur. Stand. A 75 (1971) 455. [17] R.W. Field, B.G. Wicke, J.D. Simmons, S.G. Tilford, J. Mol. Spectrosc. 44 (1972) 383. [18] J. Rostas, A. Le Floch, private communication. [19] J.B. Hasted, Physics of Atomic Collisions, Butterworths, London, 1972. [20] M. Sporoschenko, J. Chem. Phys. 49 (1968) 768. [21] J. M€ahnert, H. Baumg€artel, K.M. Weitzlel, J. Chem. Phys. 103 (1995) 7016, and references therein. [22] J.T. Blair, J.C. Weisshaar, J.E. Carpenter, F. Weinhold, J. Chem. Phys. 87 (1987) 392. [23] L. Allen, G.I. Peters, J. Phys. A 5 (1972) 695. [24] J.-M. Robbe, private communication.
Acknowledgements I should like to thank Dr. Sydney Leach for taking some of his holiday time to tidy up the manuscript in several places.
New spectroscopic observations in electric discharges through carbon monoxide Daniel Cossart Laboratoire de Photophysique Mol eculaire du CNRS and Institut de Physico-Chimie, Mol eculaire, B^ atiment 213, Universit e de Paris-Sud, 91405 Orsay Cedex, France Received 27 July 2001; in final form 25 October 2001
Abstract Two types of electric discharges were used: (a) In a supersonic expansion of pure CO gas. (b) In a U-shaped tube immersed in liquid nitrogen where either pure CO or mixtures of CO þ He, CO þ Ne, CO þ NO are introduced. In both cases, the analyzed emissions were exclusively those issuing from the negative glows of the discharges. Fourier Transform spectra were recorded in the 28 000–17 000 cm1 visible spectral region and also in the 8000–2000 cm1 infra-red interval. Comparison of the two sets of spectra showed that spectra (a) correspond to excitation of the primary species whereas spectra (b) result from excitation of final compounds produced in the positive column. In the latter case, the main new observations are: (i) Selective excitation of particular upper state vibrational levels in the E3 R –a3 P Herman system. (ii) Appearance of the C1 Rþ –B1 Rþ ð0; 0Þ Rydberg–Rydberg infrared band whereas the associated optical C1 Rþ –A1 P Herzberg transition is absent. (iii) ‘‘Anomalous’’ vibrational distribution in the ground state vibration–rotation spectra. All the above observations are concomitant with previously reported IR emissions involving very high rotational levels (up to J ¼ 120) in the ground state. To explain them, processes involving dissociative recombinations of dimer cations are suggested. Ó 2002 Published by Elsevier Science B.V.
1. Introduction Among the remaining spectroscopic problems of interest concerning carbon monoxide, one relates to the CO2þ doubly charged ion, and another to the quintet states of neutral CO. As only vibrational analyses were available on these subjects [1,2], I attempted to obtain the corresponding spectra in the gas phase. With this double objec-
E-mail address: [email protected] (D. Cossart).
tive, two special emission sources were developed, both working at low temperature in order to make easier identification of new bands in spectral regions where the known emissions of CO are very congested. The two sources, a Penning-type electric discharge in a supersonic expansion, and a Negative glow emission in a U-shaped tube immersed in liquid nitrogen, are described in [3] and [4], respectively. The supersonic jet source allowed detection of several emission lines that could belong to the CO2þ di-cation, whereas a number of newly
0301-0104/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 3 0 1 - 0 1 0 4 ( 0 1 ) 0 0 5 7 3 - 0
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observed bands remained unidentified [5]. As for the U-shape tube, it was initially built with the objective of observing the gas phase spectrum of the intersystem a005 P–a03 Rþ and a005 P–d3 D transitions of CO. Bands of the latter have been earlier detected in the 350–550 nm spectral region in a CO/Ar matrix [2] and have been tentatively assigned to the above transitions on the basis of ab initio calculation [6] on the lowest quintet states of CO. Attempts to observe these bands in the gas phase proved negative, and several reasons to justify this failure will be given here later. However, by means of the cooled U-shape tube, we observed in the Dv ¼ 1 sequence of the ground state vibration–rotation spectrum (2500–1800 cm1 ) extremely high rotational excitations (up to J ¼ 120) that likely result from þ dissociative recombination of the ðCOÞ2 dimer cation [4]. The latter emissions appeared concomitant with ‘‘anomalous’’ vibrational distributions in the ground state together with selective state level excitation, particularly the v0 ¼ 7, 11 vibrational levels of the E3 R state, and v0 ¼ 0 level of the Rydberg C1 Rþ state. Thorough examination and comparison of the spectra obtained with the two emission sources was carried out, providing assignments of the previously unidentified bands, as well as possible mechanisms to explain qualitatively the above observations.
2. Experimental The Penning-type electric discharge in a Supersonic Expansion and the negative glow emission in a U-shaped tube immersed in liquid nitrogen are hereafter labelled, respectively, SE and UST experiments. A Brucker IFS 120 HR Fourier Transform spectrometer was equipped with a Hamamatsu 1P 21 photomultiplier to record the spectra in the 28 000–17 000 cm1 visible region, and with a cooled InSb detector for the 8000–1800 cm1 IR spectral interval. About 200 interferograms were co-added in each experiment with resolutions of, respectively, 0.2 and 0:01 cm1 for the visible and IR spectral regions.
2.1. SE experiment The experimental conditions are given in [3], i.e. pumping with a 3700 l/s diffusion pump backed by a 50 m3 =h mechanical pump. Before expansion, pure CO gas was introduced at a pressure of 1 bar, whereas a pressure of about 103 mbar was measured in the ionization cell. The image of the spectrometer entrance diaphragm projected onto the expansion region was a circle (diameter d ¼ 1 cm) contiguous to the nozzle throat. In that case, only emissions during a time interval shorter than d=u, u being the speed of the isentropically expanding gas, can be observable. Assuming that h þ u2 =2 ¼ h0 , where h0 and h are the gas enthalpies per unit mass at the expansion source and at a given point in the Mach bottle, respectively, the basic kinetic theory of gases provides the following expression of the gas velocity 1=2 [7]: u ¼ ½k 2cðT0 T Þ=m ðc 1Þ , where k is the Boltzmann constant, m the molecular weight, (T0 T ) the cooling effect, and c ¼ Cp =Cv the heat capacity ratio. The latter can be roughly estimated (c ¼ 1:4) from the relation: ð3=2Þ ðCp –Cv Þ= Cv ¼ (translational degrees of freedom)/(rotational + translational degrees of freedom) ¼ 3/5 in the case of a diatomic molecule. By setting m ¼ 28, c ¼ 1:4 and T0 T ¼ 270 K, one obtains: u ¼ 750 m/s. It means that radiative transitions whose upper state lifetimes exceed d=u ¼ 13 ls cannot be detected in the SE experiments. They include rotation–vibration and optical intersystem transitions whose mean durations are often in the millisecond range. 2.2. UST experiment In contrast to the preceding situation where only rapid initial processes can be observed, the spectra recorded when using the discharge tube immersed in liquid nitrogen described in [4] correspond to emissions from final products. Indeed, before to entering the observation region (negative glow) the gases, initially introduced in the anode side at a pressure of about 4 Torr, went through a 60 cm length U-shaped positive column, the pumping way being in the cathode side. Experiments were carried out successively with pure CO,
D. Cossart / Chemical Physics 276 (2002) 167–179
CO þ He, CO þ Ne, and finally CO þ NO. In each of the three last cases, 50% flowing mixtures were used. The pressures in the cathode region were only several tenths of a Torr (except in the CO þ He case) since the major part of the gases was trapped in the positive column cooled at liquid nitrogen temperature. It should be noted that, in the UST experiments with pure CO or CO plus rare gases, the tube walls rapidly become blackened due to solid carbon deposition, whilst such a carbon deposition did not occur in the CO þ NO case, but condensation of N2 O produced in the discharge gave rise to a blue–violet coloration of the tube.
3. Results Fig. 1 reproduces parts of the emission spectra between 26 300 and 21 125 cm1 . Spectra a and b correspond, respectively, to the UST and SE emission sources with pure CO gas. The visible-toIR spectral observations (30 000–2000 cm1 ) are summarized in Table 1 showing the differences when using either the SE or the UST emission sources. Aside from those resulting from the dif-
169
ferent rotational temperatures, the main differences are the following: 3.1. Triplet transitions Bands of the d–a Triplet system involving high v0 levels (v0 P 10) appear in both the SE and UST spectra, while those of the a0 –a Asundi system appear only in the SE spectrum where the low rotational temperature strongly enhances the intensity of the first J-lines in the a03 Rþ –a3 P2 subbands. Sets of 25 and 49 such bands were observed in the SE spectra of the a0 –a and d–a transitions, respectively. Band head measurements for several of them have already been reported in the Krupenie review [8] but, as no rotational analysis was performed until now for these high v0 triplet transition bands, I give in Tables 2 and 3 the high resolution measurements for the strongest line in each sub-band. They may help the identification of CO in low temperature spectra. Fig. 2 shows, as an example, the Triplet d–a ð10; 1Þ band recorded by means of both the SE (spectrum b) and UST (spectrum a) sources with the strongest R lines indicated. Note that, while the R13 (2) and R2 (1) lines are obviously the most
Fig. 1. Fourier Transform emission spectra between 21 000 and 26 500 cm1 of CO excited in the UST source (spectrum a) and SE source (spectrum b). See text for the meanings of the SE and UST initials. Note in the UST spectrum the strong e–a Herman bands involving the v0 ¼ 7 and 11 levels, and the absence of the C–A Herzberg bands. Bands of COþ labelled CT and BJ correspond, respectively, to the Comet–Tail and Baldet–Johnson systems.
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Table 1 Summary of the spectral emissions of CO observed under various experimental conditions
Vibration Rotation Asundi Triplet Herman ngstr€ A om Herzberg Rydb–Rydb Rydb–Rydb
1
þ
XR X1 Rþ a03 Rþ –a3 P2 d3 D–a3 P e3 R –a3 P B1 Rþ –A1 P C1 Rþ –A1 P C1 Rþ –B1 Rþ E1 P–B1 Rþ
SE pure CO
UST pure CO
UST CO þ He
UST CO þ Ne
UST CO þ NO
n.o. n.o. v0 up to 20 v0 up to 25 v0 up to 12 o o n.o. n.o.
vmax ¼ 2 and 8 vh J.o v0 up to 10 v0 up to 11 v0 ¼ 7 and 11 o n.o. o n.o.
vmax ¼ 6 vh J n.o. n.i. n.i. n.i. n.i. n.i. n.i. n.i.
vmax ¼ 2 and 8 vh J.o n.i. v0 up to11 v0 ¼ 7 and 11 o n.o. o n.o.
vmax ¼ 6 and 32 vh J.o n.i. n.i. n.i. n.i. n.i. n.i. n.i.
SE: Penning-type excitation in supersonic expansion; UST: Negative glow in a U-shaped tube cooled at liquid nitrogen temperature; vh J: very high J values (up to J ¼ 120); o/n.o.: observed/ not observed. n.i.: not investigated; The most significant observations are indicated by bold face figures.
Table 2 High v0 sub-bands observed for the Triplet system of CO v00 v0
0
1
2
10
22696.29 770.46 23653.72 728.04 24592.70 667.13 25513.18 b
20981.95 1056.25 21939.39 2013.52 22878.34 952.60 23798.88 873.06 24700.96 81.64 25584.45 659.44 26449.34 524.21 27295.46 370.30 28122.57 95.65
19296.83 370.67 20254.23 328.38 21193.15 267.38 22113.73 87.77 23015.79 91.73 23899.38 974.01 24764.30 839.02 25610.38 b 26437.45 514.05 27245.27 319.66
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
3
20457.81 531.76 21359.89 435.73 22243.51 318.01 23108.37 82.84 23954.50 4029.03 24781.63 b b 25663.59 26377.53 451.42 27145.63 219.01 27893.06 965.48
4
b 21556.49 22327.81 402.57 23154.96 231.32 b 24037.26 24750.94 825.13 25518.96 b 26266.40 339.10 26992.55 b 27696.63 b
5
20725.95 99.99
22365.32 –
23923.23 – 24669.01 b 25391.70 464.95 26099.21 – 26780.25 –
Figures in cm1 correspond to wavenumber measurements of the R13 (2) and R2 (1) lines in, respectively, the 3 D3 –3 P2 and 3 D2 –3 P1 sub-bands (see text); b: blended by stronger emissions of CO or COþ .
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Table 3 High v0 sub-bands observed for the Asundi system of CO v00 v0
0
1
10 11 12 13 14 15 16 17 18 19 20 21
18002.45
16288.08 17297.03 18287.54 b 20211.80 21146.44
2
16602.41 17573.81 18525.92 19461.31 20377.34
3
16870.04 17805.36 18721.43 19619.30 20498.00
4
5
6
17094.78 17992.90 18871.31 19731.74 20579.88
18135.78 18976.00
16558.82 18229.14
Figures in cm1 correspond to wave number measurements of the Q13 ð2Þ lines in the a0 3 Rþ –a3 P2 sub-bands (see text); b : blended by stronger emissions of CO or COþ .
Fig. 2. Fourier Transform emission spectra of the d3 D–a3 P ð10; 1Þ transition band of CO excited in: (a) UST source, (b) SE source (see text).
prominent in the d3 D3 –a3 P2 and d3 D2 –a3 P1 subbands, the rotational intensity distribution is more spread out in the third d3 D1 –a3 P0 component because of the different relative line strength ratios [9]. For this reason, band head measurements are given in Table 2 only for the d3 D2 –a3 P1 and d3 D3 –a3 P2 components. It can be remarked that, because of the low rotational temperature, the intensity distributions in the ð10; 1Þ sub-bands
shown in Fig. 2 are very different from those displayed by Carroll [10] for the ð3; 0Þ band recorded at room temperature. One can obtain a rough estimation of the rotational temperatures in spectra a and b of Fig. 2 by measuring the ratio between the R13 (2) and R13 (3) line intensities in the d3 D3 –a3 P2 sub-bands. These particular peaks, the most intense in a given band, were chosen not only for this reason but, above all, because they
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represent the ‘‘quasi unperturbed’’ intensities, since the other possible way to radiatively depopulate the upper d3 D3 sub-levels, i.e the d3 D3 –X1 Rþ transition, implies violation of two selection rules. On the other hand, the spin-uncoupling is minimum for these first J -levels. This situation is confirmed by the scarcity of the corresponding observed subbands. Only one vibrational d3 D3 sub-level (v0 ¼ 4) has been indeed observed in the d–X spectra [14]. The above mentioned intensity ratio is equal to: s ¼ I½R13 ð2Þ=I½R13 ð3Þ / SJR0 ¼3 exp½F 0 ð3Þ hc=kT =SJR0 ¼4 exp F 0 ð4Þ hc=kT ¼ SJR0 ¼3 =SJR0 ¼4 exp F 0 ð4Þ F 0 ð3Þ hc=kT ; where 0 0 0 SR J 0 ¼ ðJ þ 2ÞðJ þ 1Þ=4J
is the R-branch line strength coefficient for a 3 D–3 P transition [9] and F 0 ð4Þ–F 0 ð3Þ the energy difference between the corresponding 3 D3 upper-state levels which is equal to m½R13 ð3Þ m½Q13 ð3Þ ¼ 14:52 cm1 . The s ratios were determined to be 1.17 and 1.64 for, respectively, the spectra a and b of Fig. 2 leading to the corresponding rotational temperatures in the UST and SE experiments: T 77 and 34 K. Note that the rotational tem-
perature in the UST experiment is exactly that of the liquid nitrogen bath, so that the d3 D levels are surely populated by vertical excitation of thermalized CO molecules in their ground state. Fig. 3 shows, as an example, the ð16; 3Þ band of the Asundi a0 –a system recorded when using the SE source. Note that the v0 P 10 levels were not observed with the UST source. It should be emphasized that only the a03 Rþ –a3 P2 component was detectable for this electronic transition, which is the reason why in previous work the 25 bands listed in Table 3 remained unidentified [5]. However, examination of the calculated low J line strengths for this electronic transition [9] fully explain this apparently striking observation. Similarly to the Triplet system considered above, Table 3 gives the measurements for the Q13 (2) lines. Knowledge of the high v0 level rovibronic constants for the a03 Rþ ; d3 D and e3 R states results chiefly from analysis of the vacuum UV absorption spectra [14–16], all the considered triplet-singlet transitions being spin forbidden. In that case, their intensities are borrowed essentially through spin– orbit interaction from the allowed 1 P, 1 Rþ –1 Rþ singlet–singlet transitions. An extended deperturbation treatment has been performed by Field et al. [17] providing experimental values for the perturbation matrix elements together with deperturbed rovibronic constants. Local coincidences between
Fig. 3. Fourier Transform emission spectrum of the a0 3 Rþ –a3 P2 ð16; 3Þ transition sub-band of CO excited in the UST source (see text). Note that the two associated sub-bands were not detectable.
D. Cossart / Chemical Physics 276 (2002) 167–179
triplet and singlet levels cause, in this case, irregular band intensity variations from one vibrational level to the next in the upper state. Tables 2 and 3 show, on the contrary, regular trends in the development of the Condon parabolae for the d–a and a0 –a transitions. In order to test the hypothesis that the CO triplet–triplet emissions in the SE experiment are due to vertical electron impact excitation from the v ¼ 0 ground state of the monomolecular CO, the band intensity ratios Iv0 ;v00 =Iv0 þ1;v00 þ1 in a given sequence of the d–a transition are compared in Table 4 with the corresponding Franck–Condon factor ratios: 0 2 hvd jvX ¼ 0ihv0d jv00a i=hv0d þ 1jvX ¼ 0ihv0d þ 1jv00a þ 1i ; where the Franck–Condon factors are obtained with a Morse potential for the ground state and RKR potentials for the a3 P and d3 D states [18]. Rather than normalize all the band intensities to
173
that of a reference band, I preferred to consider successive Iv0 ;v00 =Iv0 þ1;v00 þ1 intensity ratios in several sequences in order to avoid as much as possible the sensitivity variation of the detector and transmittance of the filters used in different spectral intervals. Similarly to the rotational temperatures determination in the spectra of Fig. 2, the experimental intensities were estimated from measurements of the lowest-J peak maxima in the d3 D3 –a3 P2 sub-bands. Comparison of the experimental and calculated values given in Table 4, shows that the hypothesis of vertical electron impact excitation from the CO (v ¼ 0) ground state is the most likely. In this respect, a mistake must be corrected in [5] where it has been argued that the a0 , d, e–a transitions are induced by radiative recombination of the COþ ions. This hypothesis was proposed to justify the enhancement of the neutral CO emissions
Table 4 Observed and calculated band intensity ratios in the CO, d3 D a3 P sequences Band (v0 ; v00 )
Iv0 ;v00 (arb. units)
½ðIv0 ;v00 Þ=ðIv0 þ1;v00 þ1 Þ observed
102 hvd jva i2 ¼ FC1
102 hvd jvX¼0 i2 ¼ FC2
104 ðFC1 FC2 Þ
½ðIv0 ;v00 Þ=ðIv0 þ1;v00 þ1 Þ calculated
ð10; 0Þ ð11; 1Þ ð12; 2Þ ð13; 3Þ
315 634 612 188
0.49 1.04 3.25
2.80 6.16 5.48 1.73
6.85 7.38 7.59 7.52
19.18 45.46 41.59 13.01
0.42 1.09 3.20
ð11; 0Þ ð12; 1Þ ð13; 2Þ ð14; 3Þ
193 511 564 272
0.38 0.91 2.07
1.75 4.84 5.68 3.07
7.38 7.59 7.52 7.17
12.92 36.74 42.71 22.01
0.35 0.86 1.94
ð12; 0Þ ð13; 1Þ ð14; 2Þ ð15; 3Þ
102 367 503 341
0.28 0.73 1.48
1.09 3.60 5.26 4.00
7.59 7.52 7.17 6.69
8.27 27.07 37.71 26.76
0.30 0.72 1.41
ð13; 0Þ ð14; 1Þ ð15; 2Þ ð16; 3Þ ð17; 4Þ
64 210 367 339 153
0.30 0.57 1.08 2.22
0.67 2.56 4.49 4.38 2.17
7.52 7.17 6.69 6.11 5.42
5.03 18.35 30.04 26.76 11.76
0.27 0.61 1.12 2.27
ð15; 1Þ ð16; 2Þ ð17; 3Þ ð18; 4Þ ð19; 5Þ
125 251 280 169 42
0.50 0.90 1.66 4.02
1.78 3.64 4.28 2.91 0.78
6.69 6.11 5.42 4.78 4.19
11.91 22.06 23.2 13.91 3.22
0.54 0.95 1.67 4.32
Note: The Iv0 ;v00 intensities, in arbitrary units, correspond to the peak maxima for the R13 (2) lines; The Franck–Condon factors FC1 and FC2 were calculated with RKR potentials.
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D. Cossart / Chemical Physics 276 (2002) 167–179
relative to those of the COþ and CO2þ ions when the pumping speed in the SE experiment was reduced. To explain this observation, it seems now more reasonable to suggest that, instead of favouring the neutral CO emissions, reduction of the pumping speed, i.e. pressure increase in the ionization cell, can diminish the ion densities through dissociative recombination, a process very much more efficient than radiative recombination [19]. Such a comparison between the observed and calculated intensities for the d3 D2 –a3 P1 component (the experimental intensities were those of the R2 (1) peaks) shows almost the same agreement, indicating that the d3 D2 sub-levels, like the associated d3 D3 components, relax mainly to the a3 P manifold. The situation appeared quite different for the third d3 D1 –a3 P0 component as well as for the a03 Rþ , e3 R –a3 P bands since many spin–orbit interactions involving the X ¼ 1 quantum number may occur with neighbouring A1 P state levels. While the hypothesis of vertical excitations from the monomolecular CO ground state seems now well established (at least for the d3 D levels) in the case of the SE experiment, the situation appeared different from the UST experiments. Concerning the d–a emission bands, the vibrational intensity distribution remains broad. On the other hand, it has been shown above that the d3 D levels are in thermal equilibrium with the ‘‘bath’’ (T 77 K) so that the excitation process should be the same as in the SE experiment. However, similar to the high v0 , a03 Rþ –a3 P bands which do not appear in the UST spectrum, most of the e3 R –a3 P Herman bands are also absent, except several involving the v0 ¼ 7 and 11 levels. The latter appear with high intensities, as can be seen in Fig. 1 where the ð7; 0Þ, ð11; 0Þ and ð11; 1Þ bands are indicated. In spite of their complexity, rotational temperatures in the latter transitions (T 190 K) were roughly estimated as before by determination of the J values (Jmax 7–8) for the Q3 lines of maximum intensity in the e3 R –a3 P0 sub-bands. It should also be remarked that, under the same experimental conditions, several ‘‘High Pressure’’ Swan bands of C2 are visible, i.e. those involving selective excitation of the d3 Pg , v0 ¼ 6 level. The ð6; 5Þ band is shown in Fig. 1. Note that, while the C2 ‘‘High pressure’’ and CO v0 ¼ 7, 9 Herman
bands appear simultaneously, their intensities are not correlated in different experiments. Finally, it should be remembered that the present experimental conditions are those corresponding to the excitation of very high rotational levels (up to J ¼ 120) in the vibration–rotation spectrum of the CO ground state [4]. 3.2. Singlet transitions ngstr€ om and All the expected B1 Rþ –A1 P A C1 Rþ –A1 P Herzberg bands appear in the SE spectrum of Fig. 1, but the C1 Rþ –A1 P bands with v0 ¼ 0, v00 ¼ 1–4 are completely absent in the UST spectrum. However, under the same experimental conditions (UST, pure CO and UST, CO þ Ne), the Rydberg–Rydberg C1 Rþ –B1 Rþ ð0; 0Þ transition strongly appeared at 5002:885 cm1 . Fig. 4 shows the corresponding spectra obtained by means of the UST source with pure CO and CO + Helium. The rotational temperatures for a 1 þ 1 þ R – R transition can be easily estimated from the Jmax values as T 310 and 110 K for the pure CO and CO þ He experiments, respectively. This suggests that the C1 Rþ levels are initially populated by a dissociation process inducing a rotational temperature higher than that of the ‘‘bath’’, the addition of helium generating a partial thermalization. This band was first observed by Amiot et al. [11] and Roncin et al. [12] together with the E1 P–B1 Rþ ð0; 0Þ band of another Rydberg–Rydberg transition. Note that the latter transition was not observed here. The ratio of the m3 factors (about 100) between the visible C1 Rþ –A1 P and infrared C1 Rþ –B1 Rþ transitions would suggest, at first sight, that the electronic transition moments and Franck– Condon factors are much more favourable for the C–B than for the C–A transition. This hypothesis should be rejected if one considers that, whereas the C–A Herzberg bands (upper state lifetime ¼ 1.5 ns [13]) are clearly present in Spectrum b of Fig. 1, the C–B ð0; 0Þ transition band does not appear in the SE spectra where transitions whose time constant is less than 13 ls can be observed. Moreover, the rather high rotational temperature determined above (T 310 K) is to be contrasted with that (T 77 K) in the d3 D–a3 P optical spectrum.
D. Cossart / Chemical Physics 276 (2002) 167–179
175
Fig. 4. Fourier Transform emission spectrum of the C1 Rþ –B1 Rþ ð0; 0Þ Rydberg–Rydberg transition of CO recorded with the UST source (see text) with and without addition of helium.
3.3. Vibrational intensity distribution in the vibration–rotation spectra As mentioned above, the IR vibration–rotation spectra are not detectable with the SE source since the upper level lifetimes always exceed the 13 ls limit resulting from the high gas expansion speed. The spectra shown in Fig. 5 were obtained when using the UST source. The Dv ¼ 2 sequence was considered since the InSb detector, which is sensitive down to only 2000 cm1 , would limit the observation of an extended Dv ¼ 1 sequence. The lower spectrum a corresponds to excitation of CO + Helium. Emissions from vibrational levels up to v0 ¼ 17 are visible with a maximum intensity around v0 ¼ 6. The intermediate spectrum b obtained with pure CO (or with CO þ Ne) shows two maxima for v0 ¼ 2 and 8. The number of excited vlevels is approximately the same as in spectrum a, but one observes that this is not the case for the upper spectrum c (CO þ NO) where vibrational levels were excited up to v0 ¼ 38. A first intensity maximum at v0 ¼ 6 is similar to that observed for the CO þ He case, whereas a second one appeared at v0 ¼ 32. It should be pointed out again that these ‘‘anomalous’’ vibrational intensity distributions were observed under the same experimental
conditions (pure CO or CO þ Ne, CO þ NO, but not CO þ HE where ro-vibrational distributions were thermalized by collisions with helium atoms) as those producing excitation of very high rotational levels in the Dv ¼ 1 sequence [4].
4. Discussion The above differences between the UST and SE spectra, the latter being taken as a reference of direct excitation of the CO monomer, strongly sugþ gest that dissociative recombinations of the ðCOÞ2 dimer ion play an important role in the UST experiments. In the UST source, the gases were suddenly introduced through a small orifice into the negative glow after passing through a positive column cooled at liquid nitrogen temperature. From a study of the mobility of COþ , COþ 2 and þ ðCOÞ2 in carbon monoxide glow discharges, þ Saporoschenko [20] showed that ðCOÞ2 is the principal carrier ion at small E=p0 values, where E is the electric field and p0 the gas pressure reduced to the corresponding value for 0 °C. When E=p0 increases, the ðCOÞþ 2 density decreases while those of COþ and COþ increase in a correlated manner. The 2 small E=p0 condition is particularly well satisfied in
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Fig. 5. Fourier Transform emission spectra of the Dv ¼ 2 sequence in the ground state of CO excited in the UST source (see text): (a) CO + Helium, (b) pure CO, (c) CO + NO.
the positive column of the UST source, due to the low voltage gradient and the high gas equivalent pressure, the temperature being near to that of the CO condensation. Moreover, it has been shown [20] that, under these conditions, the pressure deþ pendence of the ðCOÞ2 ion intensity is characteristic of a two-body collision: COþ þ CO ! þ ½ðCOÞ2 ; v in which v refers to vibrationally excited levels, the maximum initial vibrational energy being equal to the dissociation energy [21] of the dimer ion D0 ffi IPðCOÞ IP ½ðCOÞ2 ¼ 14:01 ð12:24 0:15Þ eV ¼ 14 270 1210 cm1 . This situation is schematically illustrated in Fig. 6 where values of the dissociation and ionization energies [21] of the neutral dimer are indicated. Possible dissociation limits are also given. Those for which selective and concomitant excitations were observed are shown with heavier traces, i.e: (a) The v 6 6 levels in the X1 Rþ ground state. (b) The v ¼ 7 and 11 of the e3 R state in the Herman system. (c) The v ¼ 0 level of the C1 Rþ Rydberg state in the C1 Rþ –B1 Rþ transition.
With the present data, it is not possible to establish correlations between the dimer and the fragment levels shown in Fig. 6 which appear in a concomitant manner, nor to determine whether the dissociations are direct or indirect through intermediate Rydberg states. Only the following qualitative remarks can be made concerning the four possible dissociation pathways pointed out in Fig. 6: (i) The very high rotational temperatures (Tr ffi 25 000 K) reported in a previous paper [4] for the CO ground state fragments appear only in the v0 6 6 levels. The greatest intensities for these high J-emissions appear in the ð4; 3Þ and ð5; 4Þ vibration–rotation bands [4], while the energy of the highest vibrational level observed (v0 ¼ 6) is Ev¼6 ¼ 12 460 cm1 i.e. just lower than the D0 disþ sociation energy of the ðCOÞ2 dimer ion. Since the higher levels should be predissociated, this observation was pointed out in [4] to support the hypothesis that the ‘‘High rotational temperature þ spectra’’ originate from dissociation of the ðCOÞ2 ion, another argument being that only dissociative
D. Cossart / Chemical Physics 276 (2002) 167–179
177
Fig. 6. Schematic representation of the ground state potential energies of ðCOÞ2 and ðCOÞþ 2 along the C–C internuclear distance with possible dissociation limits. For the latter, heavier traces indicate levels for which selective excitations were observed.
recombination of an ion in a bent electronic state can give rise to the very high rotational energies observed. On the other hand, the mean vibrational energy of the X1 Rþ , CO fragment was determined [4] as: hEv i ¼ 7296 cm1 . When considering the vibrational intensity distribution in Fig. 5(b), one observes that a maximum of re-absorption appears between the v0 ¼ 3–5 levels, whose average energy can be compared with the above mean energy hEv i. The latter can thus preexist in the CO fragments þ before the COþ þ CO ! ½ðCOÞ2 ; v associative collision, which can, in turn, provide the additional contribution to further excite these CO fragments in vibrational levels up to the v0 ¼ 6 level, the higher levels being predissociated along the C–C coordinate. Fig. 7 shows how this additional vibrational energy of the CO fragments can originate from the energy stored in the C–C stretching mode depending on the H bending angle. Nevertheless, due to the number of unknown parameters, it appeared impossible to determine a precise value for the Er =Ev ratio, and consequently for the H angle 2 ðtan2 H ¼ ðpr =pv Þ ¼ Er =Ev Þ. This is unfortunate, since an experimental value would be useful, in conjunction with the ab initio calculated ground þ state potential surface of ðCOÞ2 performed by Blair et al. [22], to estimate the r(C–C) distance for which the dissociation occurs. From energetics considerations, it has been suggested [4] that, in this case, a CO ðX1 Rþ Þþ COða3 PÞ repulsive potential surface should cross that of the dimer ion. However, though the calculation was limited to internuclear , it can be seen in Fig. 2 distances rðC–CÞ 6 2:25 A
(a)
(b)
(c) Fig. 7. Schematic representation of the ðCOÞþ 2 dimer ion showing possible C–C to C–O stretching energy transfers. (a) Near to the equilibrium position. Hcalc eq ffi 144° is the theoretical equilibrium bending angle [22]. (b, c) At high r(C–C) distances (linear geometry) in the turning points positions of: (b) the symmetric and (c) the antisymmetric stretching modes of C–O.
of the paper by Blair et al. [22], where the potential energy curves are drawn in the (r(C–C), H) coordinates, that the O–C–C–O structure approaches . linearity when r(C–C) increases beyond 2.25 A The only unambiguous conclusion that can be drawn in this particular case is that the H angle is significantly different from 180°, because of the very high rotational energy observed, so that the crossing region should be in the vicinity of the potential surface minimum of the dimer ion. We will see below that the situation should be different for the other selectively excited fragment levels observed.
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(ii) The vibrational energy of the e3 R , v ¼ 7 level is Ev¼7 ¼ 7260 cm1 , i.e almost equal to half the available vibrational energy ðD0 ffi 14 270 cm1 Þ. However, the intensity of the associated Herman bands is very much higher than that of the IR emissions considered above. Note also that the rotational temperature in the Herman bands under study, roughly estimated around Tr ¼ 190 K, is much lower than that of the CO(X) fragments determined in [4]: Tr ffi 25 000 K. A different dissociation pathway should thus be considered. The e3 R , v ¼ 11 level possess a vibrational energy Ev¼11 ¼ 10 992 cm1 , i.e about one and half times that of the v ¼ 7 level. Since the intensity and rotational temperature in the corresponding Herman bands do not differ from those in the v ¼ 7 level, one can assume that both the v ¼ 7 and 11 levels are populated through the same dissociative pathway. Although a simple means cannot be found to justify quantitatively the 3/2 vibrational energy ratio for the two e3 R observed levels, one could nevertheless explain why the vibrational energies can be different in two concomitant CO fragments. When considering that the rotational temperatures in cases (i) and (ii) are different by two orders of magnitude, and also that such a difference does not correspond to the energy difference between the COða3 PÞ þ COðX1 Rþ Þ and COðe3 R Þ þ COðX1 Rþ Þ dissociation limits, it seems reasonable to assume that the dissociating ðCOÞþ 2 ion is, in this case, quasi-linear so that the COðe3 R Þ þ COðX1 Rþ Þ potential surface crosses that of the dimer at much larger r(C–C) distances, i.e. near to the dissociation along the C–C coordinate. Figs. 7(b) and (c) are schematic represenþ tations of a linear ðCOÞ2 ion at the time when it breaks. The probability for this event is indeed maximum when the a; s C–O vibrations, considered as local modes, are in their turning positions. Continuous arrows represent the forces applied to the O and C atoms for a given turning point situation, while broken arrows represent the repulsive forces suddenly applied to the two carbon atoms. It can be easily seen that, in the symmetric case (Figs. 7(b), the vibrational energy stored in the C–C bound is equally transferred to the fragments, whereas in the antisymmetric case (Figs.
7(c)) this energy may be unequally partitioned. If one considers, indeed, a ‘‘normal’’ potential curve in the [U, r(C–O)] coordinates, it is reasonable to imagine that several quanta of vibration are more easily accepted in the repulsive than in the attractive region, because of the different slopes in the potential curve and, consequently, of the different vibrational function overlaps. Finally, one could consider an analoguous dissociative recombination of the C3 Oþ ion to explain the appearance of the ‘‘High pressure’’ Swan bands of C2 shown in Fig. 1. However, more information on the ground state potential surface and the dissociation energy of C3 Oþ is required. (iii) The last selectively excited level observed here is the C1 Rþ ð3prÞ, v ¼ 0 Rydberg level. As the rotational temperature (T 310 K) does not correspond to those in the selectively excited e3 R –a3 P Herman bands (T 190 K) or in the vibration–rotation spectra (T 25 000 K), one can suppose that another dissociative pathway should be considered, assuming that this state level is also populated through dissociative recombination of the dimer ion. The small energy difference between this level and the potential energy minimum of the þ ðCOÞ2 ground state imposes a flat dissociative pathway. On the other hand, the high level density of dissociative surfaces which can cross that of the ground state ion is hardly compatible with a selective excitation. An indirect dissociative recombination seems the most likely since symmetry selection rules have to be applied between high Rydberg and dissociative valence states. The purpose here is not to give a theoretical model for explaining this selectivity (calculations on the high Rydberg states and their dissociation limits are needed ), but to show that this selectivity indeed exists, in such a way that neighbouring states like the B1 Rþ ð3srÞ or E1 Pð3ppÞ are not involved in this process. In this way, a population inversion between the B and C states may be established so that an amplified spontaneous emission could induce a collective de-excitation accompanied by a line narrowing of the emission lines [23]. Line width measurements will be carried out in the future on the C–B ð0; 0Þ transition using the highest resolution of our Fourier Transform spectrometer.
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5. Conclusion
References
The spectra obtained with a Penning-type electric discharge in a supersonic expansion of pure CO were taken as reference spectra for the CO monomer, since it was shown that they result from direct vertical electron impact excitation. In this case, the vibrational and rotational intensity distributions show regular variations in agreement with the calculated Franck–Condon factors. In contrast, the spectra obtained from the negative glow of a U-shape tube immersed in liquid nitrogen showed several selective excitations of particular vibronic state levels that likely originate from dissociative recombinations of the ðCOÞþ 2 dimer ion through different pathways that remain to elucidate. Finally, I would like to conclude by making a comment on the possible observation of the a005 P state of CO for which this work was initially begun. The a005 P–a03 Rþ and a005 P–d3 D transition bands that Bahrdt et al. [2] thought to have observed in an argon matrix should necessarily borrow their intensities from triplet–triplet transitions unlike what was believed by Bauschlicher et al. [6] who calculated Franck–Condon factors for the above transitions as if they were allowed. In fact, under the low temperature conditions necessary to detect weak emissions in a crowded spectrum, emissions from the a005 P state are limited to those arising from the 5 P3 component, since this state is inverted [24] with a spin–orbit constant of about 30 cm1 . In that case, only mixing with 3 D3 levels could give intensity to the forbidden quintet–triplet transitions, but these perturbations should be observable in d–a transitions originating in the 3 D3 sub-states with vibrational quantum numbers higher than v ¼ 25, i.e. in a spectral region not investigated here.
[1] M. Lundqvist, P. Baltzer, D. Edwardsson, L. Karlsson, B. Wannberg, Phys. Rev. Lett. 75 (1995) 1058, and references therein. [2] J. Bahrdt, H. Nahme, N. Schwentner, Chem. Phys. 144 (1990) 273. [3] D. Cossart, C. Cossart-Magos, Chem. Phys. Lett. 250 (1996) 128. [4] C. Cossart-Magos, D. Cossart, J. Chem. Phys. 112 (2000). [5] D. Cossart, J.-M. Robbe, Chem. Phys. Lett. 311 (1999) 248. [6] C.W. Bauschlicher Jr., S.R. Langhoff, H. Partridge, J. Chem. Phys. 98 (1993) 8785. [7] R. Campargue, These d’Etat, Paris, 1979. [8] P.H. Krupenie, Nat. Stand. Ref. Data Ser. US Nat. Bur. Stand. 5 (NSRDS-NBS) (1966). [9] I. Kovacs, Rotational Structure in the Spectra of Diatomic Molecules, Adam Hilger, London, 1969. [10] P.K. Carroll, J. Chem. Phys. 36 (1962) 2861. [11] C. Amiot, J.-Y. Roncin, J. Verges, J. Phys. B 19 (1986) L19. [12] J.-Y. Roncin, A. Ross, E. Boursey, J. Mol. Spectrosc. 162 (1993) 353. [13] K.P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979. [14] S.G. Tilford, J.D. Simmons, J. Phys. Chem. Ref. Data. 1 (1972) 147. [15] G. Herzberg, T.J. Hugo, S.G. Tilford, J.D. Simmons, Can. J. Phys. 48 (1970) 3004. [16] J.D. Simmons, S.G. Tilford, J. Res. Nat. Bur. Stand. A 75 (1971) 455. [17] R.W. Field, B.G. Wicke, J.D. Simmons, S.G. Tilford, J. Mol. Spectrosc. 44 (1972) 383. [18] J. Rostas, A. Le Floch, private communication. [19] J.B. Hasted, Physics of Atomic Collisions, Butterworths, London, 1972. [20] M. Sporoschenko, J. Chem. Phys. 49 (1968) 768. [21] J. M€ahnert, H. Baumg€artel, K.M. Weitzlel, J. Chem. Phys. 103 (1995) 7016, and references therein. [22] J.T. Blair, J.C. Weisshaar, J.E. Carpenter, F. Weinhold, J. Chem. Phys. 87 (1987) 392. [23] L. Allen, G.I. Peters, J. Phys. A 5 (1972) 695. [24] J.-M. Robbe, private communication.
Acknowledgements I should like to thank Dr. Sydney Leach for taking some of his holiday time to tidy up the manuscript in several places.