From Selective to Non-Selective Vibrational Predissociation in Glyoxal van der Waals Complexes

Vibrational predissociation (VP) of glyoxal complexes formed in a supersonic expansion, has been studied in several vibrational levels of the first excited singlet electronic state Sl(1A,) of glyoxal. Light complexes as H2-glyoxal display a nice state to state dissociation. Whereas the analysis of the fluorescence spectrum, together with lifetime measurements, have shown that VP is in competition with other intramolecular vibrational energy redistribution processes in these systems, as well as inter-system crossing (ISC). Glyoxal-Ar and glyoxal-Kr behave very similarly. In particular, two isomers exist with different dissociation schemes (but similar for the same isomer of Ar or Kr-complex), and Kr does not seem to enhance ISC. Thus selectivity in the dissociation routes has to be understood as a competition between these deactivation channels.

expansion, has been studied in several vibrational levels of the first excited singlet electronic state Sl(1A,) of glyoxal.Light complexes as H2-glyoxal display a nice state to state dissociation.Whereas the analysis of the fluorescence spectrum, together with lifetime measurements, have shown that VP is in competition with other intramolecular vibrational energy redistribution processes in these systems, as well as inter-system crossing (ISC).Glyoxal-Ar and glyoxal-Kr behave very similarly.In particular, two isomers exist with different dissociation schemes (but similar for the same isomer of Ar or Kr-complex), and Kr does not seem to enhance ISC.Thus selectivity in the dissociation routes has to be understood as a competition between these deactivation channels.
The photodissociation of van der Waals molecules proceeds through the vibrational predissociation of the weaker van der Waals bond.In a diatomic rare gas complex this photodissociation is non-statistical and, among all the accessible levels of the diatomic the dissociation leads only to the highest energetically accessible levels of this diatomic.In a polyatomic complex the question of the selective or non-selective rupture of the van der Waals band is of importance since it connects with the selective infrared photodissociation. 2 In such experiments the energy is first localized in one highly excited vibrational overtone, then causes the rupture of a weaker bond.
We studied here the complexes of a polyatomic molecule, trans- glyoxal-(CHO--CHO) with H2, D: and various rare gases.
We observed two opposite behaviors" 1) The selective, mode to mode dissociation of H2, D2 complexes for which dissociation we give a propensity rule.
2) The near random dissociation of heavy (Ar, Kr... glyoxal) com- plexes where the randomization can be explained in terms of vibra- tional redistribution within the complex.

EXPERIMENTAL METHOD
The photodissociation is observed in the first excited state 1Au of the complex and the molecule.The electronic and vibrational states of the complex are very similar to those of the molecule (with in addition the van der Waals modes) as the distant rare gas only slightly perturbs the polyatomic.
The complexes are formed in the supersonic expansion of helium + rare gas (a few %) + glyoxal.The initial (complex) and product species are analyzed through their excitation and their temporally resolved fluorescence spectra.

GlyoxaI-H2 complexes
As an example we show in Figure 1 an energy resolved fluorescence excitation spectrum 0t H2-glyoxal and D2-glyoxal in the vicinity of the 8o transition.Five distinct bands (a, b, c, d and e) clearly belong to a van der Waals mode progressions as evidenced from the H2, D2 isotope effect, band b being the van der Waals origin.
In the fluorescence excitation spectrum, each vibronic transition of the free glyoxal molecule is accompanied by satellite bands assigned to the glyoxal-(H2)l complex (all these features are absent in pure He expansion at the same conditions and their pressure dependance ascertains they correspond to a glyoxal-(H2) complex.However, no additional bands could be seen corresponding to glyoxal-(H2)2 or glyoxal-(H2)3).The spectral intervals between a reference line 0t the absorption band 0t the free glyoxal and its van der Waals features are identical (within +/-1 cm-x) for all the vibronic transitions, which implies that the vibrational frequencies of glyoxal are practically non-affected by complex formation.Moreover, no bands correspond- ing to the rotational excitation of the H2 component (J"= 0 J'= 2 or J' 1-J' 3) have been detected...-----v (cm-) FIGURE 1 State selected (0g emission of glyoxal fragments) fluorescence excitation spectra of glyoxal-D21(upper trace) and glyoxal-H2 (lower trace) following excitation in the region o[ the 80 band o[ glyoxal.Glyoxal was expanded in a mixture of 5% D2 or H2 in Helium at 15 atm through a 32 I nozzle.The peaks a, b, c, d, e are attributed to the same vibrational bands for both complexes, glyoxal-H2 and gloxal-D2.
We have now examined the dissociation scheme and made first the following observations.
We have observed that for all vibrational levels under study: (i) the vibrational relaxation of free glyoxal molecules is negligible in our experimental conditions; (ii) the excited van der Waals complex dissociates in a much shorter time scale than the lifetime of the glyoxal 1Au state (0.8-2 ixs) 3 and than the collision time interval--from fluorescence decay measurements, an upper limit of the order of 15 ns may be established for the dissociation time; (iii) the vibrational dissociation populates lower vibronic levels of the free molecule, the population distribution being identical for all the van der Waals satellites of a given vibronic transition, except for the 1o transition for which the distribution ratio 7x/0 has a maximum for feature c. --1--:2 For the 8070 transition, the resolution was too low to check this assumption.
From the intensity distributions in the fluorescence spectra recorded under the excitation of a given vibronic state of the complex (labelled as ,"I7"), the branching ratios for different dissociation channels populating the free molecule states V k W may be roughly estimated.
They are given in Figure 2 (for the and 00 transitions, we give the average distribution over the five features).Since the 2 to 71 500 Olyoxal Glyoxal-H 2 complex E _8 --5 process is highly efficient, the dissociation energy ot the complex cannot exceed 230 cm-1.As can be seen from Figure 2, most of the energetically accessible states are not populated at all: as in the case of tetrazine, 4'5 vibrational predissociation in glyoxal is a highly selec- tive process.
Interesting conclusions may be drawn from the behavior of combi- nation states: 11 and 12.For the former, the predominant channels are 1.1_.. 51 and 11__ 8171, while for the single mode states we have: 1...0 and 1_.71.One can consider that the dissociation takes place by the individual energy transter either from the 8 or the 5 mode to the dissociation continuum, both processes having similar rates.This is what we could call the "spectator model," mode 8 remaining a spectator while mode 5 dissociates to the con- tinuum and vice-versa.There is, however, a non negligible 1il 0 channel that cannot be understood this way.The spectator model is further verified by the dissociation of 1ff2 which gives fragments in the 8171 state mainly (the other levels are uncertain and poorly filled), while if2 gives 71.The dissociation of the 1-72 state would thus consist in the transfer to the continuum of one quantum o the 7 mode, much more efficient than the transfer of one 8 quantum.
The high efficiency o 2 to 71 process (AE =233 cm-1) is consistent with the energy gap law.On the other hand, the energy gap law will predict that level 1 should predissociate faster than 1 (AE 509 and 735 cm -1 respectively) and this is in disagreement with what we see.One possible explanation of this behavior comes from the analysis of normal modes in glyoxal.If we suppose that the H2 molecule is bound on the C2 axis out of the plane of glyoxal, 6 then the van der Waals stretch vibration will be more efficiently coupled to the out ot plane vibrations (8 and 7) than to the in-plane vibrations like mode 5, allowing an efficient energy transfer between intra and inter- molecular modes.These observations can be rationalized when one observes that the connected channels involve only a 1 or 2 quantum change (seldom 3).

Dissociation of argon, krypton-glyoxal complexes
The heavier rare gases are more polarizable hence they complex more tightly to glyoxal. 9Moreover we have shown the existence of multiple potential wells, i.e., the formation of two van der Waals conformers, which we shall call C and C'.From the fluorescence spectra of the dissociated species we have bracketed the dissociation energies" 233 crn -1 < DC(Ar) < 466 cm -1 DC'(Ar) < 240 cm -466 crn -x <DC(Kr) < 735 crn -DC'(Kr) 250 cm -We have observed that, aside from the vibrational predissociation there are two competing non-radiative deactivation channels for the complex.The resonant emission observed from the 0 complexes in the 1Au electronic state is much shorter (110 ns for Kr, 140 ns for Ar) than the 2 Ixs glyoxal emission.We have shown7 that this 0 deactiva- tion proceeds through intersystem crossing (dissociation into the -Av " 1000 2000 FIGURE 3 Fluorescence emission from the complexes: (a) resonance 0 emission of C (Kr), (b) resonance 0 emission of C (Ar), (c) emission resulting from 7 2 excitation of C(Kr).The left peak corresponds to the reference scattered laser light, (d) reference glyoxal 0 emission.triplet manifold of glyoxal).When the complexes are excited below the dissociation limit, with some vibrational excess energy, the reso- nant emission is no longer observed as seen in Figure 3.Here the 2 complex of krypton was excited, the sharp structure of resonant emission (as seen for 0) does not appear starting from the excitation energy, but a broad emission is formed in the 0 emission range.This behavior is characteristic of vibrational energy randomization where the if2 level is perturbed by the high density of the low frequency (c.30cm-1) van der Waals modes.This was further confirmed by the decay characteristic of this 2 level, very similar (110 ns) to the decay.The onset of such randomization is rapid and proceeds within the laser pulse, much faster than intersystem crossing, as must be occurring in the Ar-tetrazine complex. 8elective dissociation occurs when one (or a few) among the dissoci- ation pathways is faster than the others and than randomization.The dissociation is not as selective as for H2-complexes (Figures 4 and 5).However in a narrow energy range, some discrimination in the routes is retained and level 121 has not been seen in any dissociation channel,

FIGURE 2
FIGURE 2  Vibrational predissociation pathways of selected complex levels XmY.onto free glyoxal A. fluorescent levels X 'Y The relative population ratios are indicated by the niiddle figures.