Radiative lifetime of the HCO B̃2A′ state

13 May 1994

ELSEVIER

CHEMICAL PHYSICS LETTERS

Chemical Physics Letters 222 ( 1994) 245-249

Radiative lifetime of the HCO fi 2Af state Ying Jen Shiu, I-Chia Chen Departmentof Chemistry National Tsing Hua University,Hsinchu 30043, Taiwan,ROC Received 10 December 1993; in final form 14 March 1994

AhStlWt The fragment HCO, produced from acetaldehyde by photolysis at 300 nm, was detected by laser-induced fluorescence of B ‘A’g ‘A’ transitions. The fluorescence lifetimes of the HCO B state decrease rapidly as the photolysis energy increases, indicating strong predissociation of the B state. The radiative lifetimes of B are in the range 70-90 ns for (0,0,O) K’=O states and 42-50 ns for K’= 1 states. For all measured vibrational levels, the K’= 1 states have smaller average lifetimes than the K’=O states. Correction over individual lifetimes is necessary to obtain the state population if the fi state is excited. The state B (0, 1,O) has a shorter average lifetime than the states ( 1, 0,O) and (0,0,2), despite having a lower energy.

1. Introduction The laser-induced fluorescence spectrum of HCO fi *A’-g *A’was first observed by Sappey and Crosley [ 1 ] and Adamson et al. [ 21. The intense signal allows us to detect the formyl radical in this way as well as in the rapidly predissociating A state. In flames, the formyl radical is an important product of combustion of hydrocarbons. It is also the primary product of photodissociation of acetaldehydes. To study the mechanisms and chemical kinetics of a flame, we must know the population distribution of the formyl radical. Laser-induced fluorescence (LIF) is a sensitive means to indicate the relative population by comparison of the total fluorescence intensity of individual quantum states. However, for polyatomic molecules, either complicated interactions among states of different electronic spin or curve crossing to other states may affect the lifetimes of different rovibrational states [ 31. Previous studies [1,4]have shown an abrupt decrease in the fluorescence quantum yield of the HCO B state. The lifetime of various vibrational states would reveal any mode specificity 0009-2614/94/$07.00

that promotes predissociation. Various lifetimes of rovibronic states may make it difficult to obtain an accurate population distribution directly from the experimental LIF intensity of polyatomic molecules or radicals. Sappey and Crosley [ 1 ] measured the lifetimes of the~statelevels(O,0,2)and(l,O,O)tobe22f5 ns. The lifetime of B (0,0,O) is 43 + 2 ns according to measurements in the same laboratory with extrapolation on a Stem-Volmer analysis of quenching data to zero pressure [ 5 1. In our experiments we cooled acetaldehyde in a supersonic jet to avoid spectral congestion, and photolyzed it at a wavelength of 300 nm. Assignments of vibrational states and rotational analyses appear in ref. [ 41. We measured the radiative lifetimes of individual rotational states of the lowest six vibrational levels, limited by the laser resolution, of the HCO B state, under nearly collisionfree conditions. For these states, the average lifetime of K'=0 states exceeds that of R = 1 states. Scatter in lifetimes must be considered in determining the population of individual HCO rovibrational states from LIF intensities.

0 1994 Elsevier Science B.V. All rights reserved

SSDZOOO9-2614(94)00343-O

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Y.J. Shiu, I-C. Chen /Chemical Physics Letters 222 (1994) 245-249

2. Experimental The radiative lifetimes of rovibrational states of HCO B were recorded for the transitions B ‘Al-2 *A’. The formyl radical was produced from photolysis of acetaldehyde. The experiments are described in detail in ref. [ 41, therefore here only cursorily. Acetaldehyde in helium gas, 7O/6, total pressure l-2 atm was expanded in a supersonic jet and was photolysed near the outlet of the pulsed nozzle (operating at 10 Hz). The background pressure of the chamber was maintained less than 4 x 1O-’ Torr. The frequency-doubled output, 7-8 mJ at 300 nm wavelength, from a dye pumped by a Nd : YAG laser was used to photolyze acetaldehyde. The products are mainly HCO and CH3 [ 6 1. The output of another dye laser pumped by a XeCl excimer laser was frequency doubled (230-260 nm, x0.6-1.5 mJ) in a BBO crystal, and used to excite the HCO to the B state. The average duration of pulses of the dye output was R 15 ns with ? 2 nm jitter originating from the pump laser. HCO fluorescence was detected by a rapidly responding x 2 ns photomultiplier tube (EM1 9828). To avoid spectral congestion, the probe beam interacted with the jet 6-7 mm beyond the photolysis beam to let the product HCO become both rotationally and vibrationally cooled. The optimal delay between the two laser pulses was 3.5 us; this arrangement allowed us to separate the intense scattered light induced by the photolysis beam from that of the probe beam. The decay curves of HCO fluorescence were recorded and averaged over 100-200 shots in a digital oscilloscope (LeCroy 9450,350 MHz bandwidth) and were sent to a microcomputer via a GPIB connection for data processing and analysis. From each HCO decay curve was subtracted the background signal taken by blocking the photolysis beam with all other conditions kept unaltered. These background curves contain fluorescence of acetaldehyde and scattered light from the probe beam. The fluorescence intensity from the parent molecule, at the probe beam wavelength, was blocked completely by three cutoff filters (Schott 290, 320). 3. Results and discussion The fluorescence decay curves of rovibronic states of the B state were measured from vibrational levels

in the energy region from (0, 0, 0) to (0, 1, 1). Vibrational levels above the (0, 1, 1) state have much shorter lifetimes than our instrumental resolution, 5 ns, limited by the jitter of the excimer laser and the response time of the photomultiplier. The experimental fluorescence decay curves of B (0, 0, 0) NK= 4,,+ 50 and 4i + 5, states and the fits are shown in Figs. 1 and 2. Those decay curves with lifetime

0

300

200

100

400

Time/ns Fig. 1. Experimental fluorescence decay curves and fits for HCO B(O,O,O) (a)4,,+5,,and(b)4,+5,states.Thetittedl/edecay lifetimes are 70 and 50 ns, respectively. Both curves were fitted by single exponential functions. The measured transitions are (a) QR,,(~)+QR&~)at 38701.15 cm-’ and (b) QR1(3)+QR,(4) at 38693.10 cm-‘.

1”

100

150

200

250

300

Time/ns Fig. 2. Experimental fluorescence decay curves and tits for HCO B (0, 0, 1) (a) lo and (b) l1 states. The experimental curves were deconvoluted from the laser lineshape to give fitted decay lifetimes 54 and 18 ns, respectively. The measured transitions are (a) UP,, at 39754.96 cm-’ and (b) nPo(2) at 39769.56 cm-‘.

Y.J. Shiu, I-C. Chen /Chemical Physics Letters 222 (1994) 245-249

greater than 50 ns were fitted by a single exponential function by means of a Marquardt non-linear leastsquares method [ 7 1. For more rapid decay, the curves were deconvoluted from the laser lineshape and fitted according to the same method. The derived lifetimes of the lowest six g vibrational levels are shown in Fig. 3. Level (0, 1, 1) could have a lifetime even shorter than our fitted values, l-2 ns. The error of the fitted lifetimes is + 4 ns ( 1a). Level (0,0,2 ) has an average lifetime x 15 ns for K’= 0 states and 2-7 ns for R= 1 states. For all of the measured vibrational states, the K’=O states have a longer average lifetime than the K’= 1 states (Figs. 3 and 4). The vibrational level (0, 1, 0), with lifetimes 5-10 ns (K’ = 0 ) and l-3 ns (K’ = 1) , lies at lower energy than the (1, 0, 0) and (0, 0, 2) states, but has a larger

: g1003 E

2 $o-

(001)

(000) (

(010) -

(011)

i i i .

0 0

(100

(002)

1

1,,,,1,,,,1..,,1,,,.1,,,,,,,,,,,,,,1 0 KJoo

moo

3ooo

Wavenumber/cm-’ Fig. 3. Lifetimes of vibrational levels of the HCO B state. The bscissa axis denotes the energy above the vibrational zero point of the B state. ( 0 ) K’= 0 states, ( 0 ) K’= 1 states.

0

2

4

6

8

10

Rotational Quantum Number N Fig. 4. Lifetimes of rotational states of HCO B (0, 0, 0). (0 ) K’=O states, (0) K’= I states.

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(faster) average decay coefficient. Adding one quantum in the second vibrational mode increased the decay rate. In Fig. 4, the (0, 0, 0) state shows distinct lifetimes for K'=O,70-90 ns, and K'= 1, 42-50 ns, states and small dependence on the rotational quantum number N. The fitted l/e lifetimes, r, of the B (0, 0, 0) rotational states and the assigned transitions are listed in Table 1. The fitted lifetimes for the same upper states, measured via other transitions, show some variation. Limited by the laser resolution and the Doppler width of molecules in the jet, most observed lines were blended to a certain extent. Slight overlap with K’= 1 or 2 states decreases the lifetime. Variation of jet temperature would alter the ratio of contributions from distinct K'states to the measured transitions and cause some distortion of the measured values. In Table 1, the (0, 0,O) 8. state, which has a smaller lifetime than other K’=O states, may either be coupled to some unknown state or overlap an unassigned R#O state. In this work, the transitions that have larger intensities and little overlap were chosen. For K’# 0 levels, the two asymmetry components do not show obvious lifetime differences as shown in Table 1. From the b type transitions, where the two spin components are better separated, the J=N+ 4 states have slightly longer lifetimes than the J=N- f state, but the deviation is small ( d 20). The formyl radical is a nearly prolate symmetric rotor. The quantum number K is defined approximately as the projection of the total rotational quantum number N along the CO bond, the a axis. Increasing rotational motion about this axis tends to lead to predissociation, as indicated by the experimental data. The K’ = 0 states are coupled slightly less to predissociation than the K'>O states, just as the HCO A K= 0 states are coupled less strongly to the x continuum. There are several mechanisms that could affect the lifetimes. The irregular spacing of the K stacks found by Adamson et al. [ 21 may be due to interaction with a quartet electronic state [ 8,9] near the B state. The c state with its predicted linear equilibrium geometry, but which is not observed using the LIF technique [ 1,2,4], may couple to the B K'>0 states. Most likely, Coriolis coupling to the continuum of the A ‘A“ state is the main lifetime shortening mechanism, since the B lifetime exhibits strong K' dependence (a type, a K2)and also decreases slowly

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Y.J. Shiu, I-C. Chen /Chemical Physics Letters 222 (1994) 245-249

Table 1 Radiative lifetime of HCO B ‘A’( 0,0,O) Energy (cm-‘) 38714.45 38713.97 38713.10 38710.40 38709.50 38709.00 38708.52 38706.88 38704.76 38702.05 38701.15 38700.66 38700.44 38699.56 38699.20 38697.75 38697.31 38694.8 1 38693.10 38692.64 38691.14 38689.17 38685.79 38685.05 38681.70 38680.36 38676.62 38675.10 38672.40 38671.95 38670.76 38669.33 38668.93 38666.46 38662.90 38660.81 38658.04 38652.91 38647.43 38641.22 ‘8’A’(O,

7 (ns)

50 42 45 51 43 50 45 46 48 50 70 77 75 78 76 90 61 57 50 54 56 69 53 79 60 78 53 87 47 69 85 79 79 91 85 44 90 82 82 86 O,O)-x’A’(O,O,

O),“%JK~(N”).

KS

(panty)

3d-)+u+) &(+I 1,(-l II(+) II(+) 2,(-l A(-) 31(+) 41(-l 5,(+) 40(+)+50(-) 30(-J 60(+) 20(+) 70(-l lo(-) go(+) 90(-)+3,(f) 41(+)+5,(f) Oo(+)+31(+)+4,(f) 6,(f)+3,(f) 10(-)+41(+) II(*) 20(+) II(+) 30(-J 2,(f) 40(+) 3,(*)+10(-) 31(+)+10(-) 20(+) 50( - ) 30(-) 40(+) ho(+) 5,(+) 30(-l 40(+) 50(-l 60(+)

J’

Transition a

3.5+2.5 1.5 1.5 1.5 0.5 2.5 1.5 2.5 3.5 4.5 3.5,4.5+4.5, 5.5 2.5,3.5 5.5,6.5 1.5,2.5 6.5, 7.5 1.5 7.5, 8.5 8.5,9.5 + 2.5, 3.5 3.5,4.5+4.5, 5.5 0.5+2.5,3.5+3.5,4.5 5.5,6.5+2.5, 3.5 0.5, 1.5+3.5,4.5 0.5, 1.5 1.5,2.5 1.5 2.5,3.5 1.5,2.5 3.5,4.5 2.5,3.5+0.5, 1.5 3.5+ 1.5 1.5,2.5 4.5,5.5 2.5,3.5 3.5,4.5 5.5, 6.5 4.5,5.5 2.5, 3.5 3.5,4.5 4.5,5.5 5.5,6.5

wd2)+%3(1) -o(l) -o(O) RQo(l)b

RQo(llb RQo(2)b RQo(2)b RQo(3) RQo(4) RQo(5) Q%(3)+%(4) Q&G) QW5) Q%(l) QM6) QRJW Q%(7) QM8) +RPo(4) QR,(3)+QR,(4) QPo(l)+QR,(2)+QR,(

3)

QRl(5)+~d2) QPo(2)+RPo(5)

QQ~(t) QPo(3) QPl(2) QPo(4) Qpl(3) QPo(5) QP1(4)+PQ~(l) QP1(4)+PQ~(l) ‘Q1t2) *o(6)

‘Q,(3) ‘Q,(4) QPo(7) QP~(6) ppI (4) pp1(5) ‘P,(6) pp1(7)

b Split by spin-rotational interaction.

as the rotational quantum number, N’, increases for the same K’ states (b type, a N- K) , as shown in Fig. 4. The measured lifetimes of levels (0, 0, 2) and ( 1, 0,O) for K’= 0 states are all shorter than 20 ns, smaller than the value, 22 + 5 ns, observed by Sappey and Crosley [ 1 ] using the fluorescence decay method. However, our data show that the K’= 1 states have

lifetimes shorter than 5 ns. For the (0, 0,O) level, the lifetime derived by Stern-Volmer analysis of collisional quenching experiments [ 5 1, seems too short for a K’=O state, but comparable to that of K’= 1 states. Under thermalized conditions, Meier et al. [ 5 ] may have excited multiple HCO rotational states which include K’# 0 states. The (0, 1,O) level has a shorter average lifetime than the (0, 0, 1) and ( 1, 0,

Y.J. Shiu, I-C. Chen /Chemical Physics Letters 222 (1994) 245-249

0 ) levels (Fig. 3 ) , indicating vibrational mode specificity for the predissociation of the B electronic state. The second vibrational motion ( v2) is assigned primarily to the HCO bending as suggested by Cool and Song [ lo] and Adamson et al. [ 21; increasing or decreasing the HCO angle deformation produces a shorter lifetime. The experimental data implies that the v2vibrational mode in B acts as a promoting mode to predissociation. Further experiments with a laser of greater resolution would avoid some problems of spectral overlap and be able to yield the lifetimes of states of higher values of K’ and IV’.This information would be helpful to determine the population distribution of HCO from dissociating aldehydes or in hydrocarbon flames, and would enable an improved description of the interaction of B to either the quartet electronic state or Rydberg states and the predissociation mechanism.

Acknowledgement

We are grateful to Professor Robert W. Field for valuable comments. We thank Professor Yuan-Pem Lee and his group for helpful discussions and the loan

249

of some instruments and the National Science Council of the Republic of China (contract No. NSC 820208-M-007-053) for support.

References [ 1 ] A.D. Sappey and D.R. Crosley, J. Chem. Phys. 93 ( 1990) 7601. [ 21 G.W. Adamson, X. Zhao and R.W. Field, J. Mol. Spectry. 160(1993) 11. [ 31 I. Garcia-Moreno, E.R. Lovejoy and C.B. Moore, J. Chem. Phys. 98 (1993) 873. [4]Y.J. Shiu and I-C. Chen, J. Mol. Spectry., submitted for publication. [ 5] U.E. Meier, L.E. Hunziker and D.R. Crosley, J. Phys. Chem. 95 (1991) 5163. [6] A. Horowitz, C.J. Kershner and J.G. Calve& J. Phys. Chem. 86 (1982) 3094; J.-C. Loison, S.H. Kable and P.L. Houston, J. Chem. Phys. 94 (1991) 1796. [7] W.H. Press, B.P. Hannery, S.A. Teukolsky and W.T. Vetterling, Numerical recipes (Cambridge Univ. Press, Cambridge, 1986). [ 81 K. Tanaka and E.R. Davidson, J. Chem. Phys. 70 (1979) 2904. [9] K. Tanaka and K. Takeshita, Chem. Phys. Letters 87 (1982) 373. [ lo] T.A. Cool and X.-M. Song, J. Chem. Phys. 96 (1992) 8675.