S1S0 fluorescence excitation and dispersed fluorescence spectra from molecular beams containing both trans‐ and cis‐glyoxal have been used to extend the characterization of the 1A1 (S0) and 1B1 (S1) states of cis‐glyoxal. Explorations using both effusive and supersonic beams with rotational temperatures ranging from 350 to 30 K have revealed no conditions where cis can be pumped (S1S0) without simultaneous excitation of trans. Selective cis excitation at low beam temperatures is hampered by highly efficient cistrans conformational interconversion in the molecular beam expansions. Under conditions of optimal cis:trans contrast (cool expansions with Ar carrier gas), four new S1S0cis absorption bands (510,520,610, and 720 ) are identified, yielding cis frequencies ν5 =303 cm1, ν6 =713 cm1, and 2ν7 =688 cm1. Single vibronic level fluorescence spectra have been obtained from the levels 00, 51, and 61 of cis‐glyoxal, from which values of two cisS0 fundamentals are newly established: ν4 =826 cm1 and ν′′6 =1049 cm1. Previous assignments of ν4 and ν′′8 are shown to be incorrect and

ν8 now joins the list of unknown frequencies. The 1B11A1 system of cis‐glyoxal contains forbidden transitions, vibronically induced by Δv=±1 changes in the a2 mode ν6. A remeasurement of the cistrans energy separation in the ground electronic state gives ΔH=1350±200 cm1, matching to within experimental uncertainty a previous experimental determination. As an aside, the trans‐glyoxal fundamental ν′′3 =1352 cm1 has been obtained from observations of the trans 301 and 301510 transitions. With this addition, all transS0 fundamentals have now been directly measured.

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L. G. Anderson, Ph.D. thesis, Indiana University, 1972.
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25.
C13S20M50 Glass Capillary Array, Galileo Electro‐Optics Corp., Galileo Park, Sturbridge, MA 01518.
26.
Series 9 High Speed Axial Flow Solenoid valve, General Valve Corp., 202 Fairfield Rd., Fairfield, NJ 07006. (We wish to thank Professor J. D. McDonald for calling our attention to these handy little valves).
27.
The detection system was triggered so that the charge integration interval began 200 ns before the laser pulse. Therefore, all fluorescence as well as any scattered laser light was detected.
28.
DT2801/5716 data acquisition board, Data Translation, 100 Locke Drive, Marlboro, MA.
29.
The true fraction of cis00 fluorescence passed by our cutoff filters is probably greater than 24%, since the latter value represents the sum of contributions from only the strongest peaks in the cis00 fluorescence spectrum (after correcting each for filter transmission). It is safe to assume that cis00 molecules fluoresce weakly at many additional wavelengths, giving rise to many smaller (but as yet unobserved) peaks in the 00 emission spectrum. Most of this additional fluorescence will lie well to the red of our filter cutoff and will be passed by the filters, there by increasing the fraction of total cis fluorescence detected. It is interesting to note that when we tried to calculate the fraction of trans00 fluorescence passed by our filters in the manner described above (i.e., by including contributions from only the major peaks in the trans00 fluorescence spectrum), we arrived at a value of 2.5% which is three times smaller than the true value (7.2%) obtained by direct experimental measurement.
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R. K.
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32.
Trans310 emission has been observed after pumping the trans00 level in a cold molecular beam. The (000−310) displacement is 1352±1 cm−1 and the 310 intensity is 160 times smaller than the 000 intensity [D. J. Krajnovich, K. Butz, H. Du, and C. S. Parmenter (unpublished results)].
33.
The asymmetric rotor band contour program was originally written by Professor Louis Pierce and was modified to run on a VAX 11/780 by Dr. D. V. Brumbaugh. A copy of the program and instructions for its use were kindly provided to us by Professor D. H. Levy and Ms. Cheryl Morter.
34.
While the overall agreement between the experimental and calculated cis type C rotational contours is generally good, there is one discrepancy. The intensity of the large “spike” just to the red of the band origin is nearly a factor of 2 lower in the experimental type C contours than in the computer simulated type C contour. The reason for this discrepancy is not clear. Power saturation is, however, an unlikely culprit, since the experimental and calculated cis type A rotational contours are nearly identical. (Note from Fig. 3 that the cis000 and cis601 absorption bands have comparable intensities. They should, therefore, be similarly susceptible to saturation effects).
35.
There are subtle distinctions between our δH, Currie and Ramsay’s δH, and the δH of Osamura and Schaefer. Among the three, the number calculated by Osamura and Schaefer is most unique, being really the difference between the bottom of the cis and trans wells on the torsional PES. A more subtle difference exists between the experimental δH determinations. Currie and Ramsay monitored the absorptions occurring in two bands, the cis000 and the trans triplet 201 bands. We, on the other hand, monitored the cis000 band and the trans000+711+722 bands. Thus, our determination intrinsically differs from Currie and Ramsay’s by inclusion of trans71 and 72 populations. It is probable that the true differences are less than the errors.
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A. R. H.
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and
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F. W.
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K.
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,
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(
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) (electron diffraction determination of trans equilibrium structure). For experimental determinations of the cis equilibrium structure, see Refs. 19 and 20.
39.
Note that our reassignments concerning ν8 and ν6 unrelated to the fact that two axis conventions are sometimes used in connection with the C point group (i.e., depending on whether the x or z axis is chosen to coincide with the C2 symmetry axis). The use of the b1 and b2 symmetry labels depends on which axis convention is chosen. Thus, the vibration referred to as ν8 (b1) by Ramsay and co‐workers would be labeled b2 under the other axis convention. The ν6 vibration, on the other hand, has a2 vibrational symmetry under either axis convention, and this is the only vibrational symmetry that can explain the relevant experiment facts.
40.
K. W. Butz, J. J. Johnson, D. J. Krajnovich, and C. S. Parmenter (to be published).
41.
The value χ57 = −8.3 cm−1 was derived from a least‐squares fit to the positions of seven trans energy levels of the type 517n (with n<8). The 51−7n level positions were read from several trans‐glyoxal SVL fluorescence spectra measured in a cold molecular beam.
42.
G. H.
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D.
Kivelson
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(
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45.
Each asymmetric rotor level, JKa,Kc, of cis‐glyoxal has a nuclear spin statistical weight of either 1 or 3, owing to the presence of two identical hydrogen atoms in the molecule. The weighting factor is 1 if Ka and Kc are both even or both odd, and 3 otherwise.
46.
A copy of this table was obtained from the Depository of Unpublished Data, National Science Library, National Research Council, Ottawa, Canada.
47.
D. J. Krajnovich, K. W. Butz, H. Du, and C. S. Parmenter (unpublished).
48.
The importance of glyoxal‐glyoxal collisions in the conformational interconversion might explain one curious experimental result. Namely, that we observed better cis:trans contrast when we simultaneously (i) used a higher pressure of carrier gas, and (ii) operated on the early peak in the pulsed valve fluorescence profile. The importance of (i) is that the glyoxal beam is more dilute. (Recall that the glyoxal partial pressure is held fixed at ∼10 Torr for most of our experiments). The importance of (ii) is that the stagnation pressure has not yet built up to its final value. Taken together, (i) and (ii) suggest that similar high‐contrast spectra would be obtained under steady‐state (or cw) conditions if the same dilute glyoxal mixture were expanded at a lower total stagnation pressure.
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