Consecutive Vibrational Redistribution and Predissociation Processes in s-Tetrazine-Argon Van Der Waals Complexes

s-Tetrazine argon complexes T-Ar, (n 1, 2) are formed in a supersonic expansion of argon seeded with s-tetrazine. The expansion was conducted through a nozzle of 50 or 100 pm with an argon stagnation pressure between and 1.5 bar. From spectrally resolved measurements it is clear that vibrational redistribution processes as well as vibrational predissociation processes take place after SVL excitation within the complex. From rise and decay time experiments it can be concluded, that after excitation of the 6a complex level, the above mentioned processes are consecutive and not parallel. It appears that the out of plane mode 16a couples with the Van der Waals stretching mode. The predissociation rate of the 16a complex is observed to be 2.3 x 109 -t.


INTRODUCTION
In a supersonic expansion of s-tetrazine seeded in argon, Van der Waals complexes are created during the cooling process. The weakly bound complexes are stable in the region after the nozzle and can thus be studied by laser induced excitation and fluorescence spectroscopy. [1][2][3] The T-Ar, (n 1, 2) complexes as well as the parent molecule tetrazine can be studied in the supersonic jet under identical conditions by selective excitation. A good comparison then is possible with the known SVL spectra of s-tetrazine in a vapor at room temperature under isolated conditions and in the single collision regime. 4 '5 After SVL excitation of the complexes, resonance emission from the prepared level is observed in combination with vibronic level emission from other modes which is attributed to either complex emission or to emission from the parent molecule. 2'3 In this paper we will attribute these results to vibrational relaxation and vibrational predissociation processes. The different channels are followed in time by means of fast decay and rise time experiments. The characterization of the fluorescence emission and excitation spectra is available in literature. 1'4'5 In a vapor at room temperature (0.5 torr) no vibrational relaxation processes are observed in the different vibrational levels studied. On the other hand the decay times observed in the excited state for the different single vibronic levels up to 1000 cm -1 change from < 100 ps up to 1.5 ns. It is assumed that this is due to internal conversion processes followed by photochemical decomposition.

SUPERSONIC JET SPECTROSCOPY
In the supersonic jet experiments with a nozzle diameter of 50 and 100 Im and an Ar stagnation pressure of 1.2 bar with 0.03% stetrazine vibrational relaxation processes in the tetrazine molecule itself are not observed (i.e., < 0.5 %) in a region after the nozzle from 0 to 5 mm. The dimension of the area excited by the synchronously pumped CW dye laser and observed by a high resolution monochromator in all experiments was of the order of 50 x 50 Im.
In contrast to the observation with the molecular species we observed efficient vibrational relaxation and vibrational predissociation effects when the Ar Van der Waals complex species were excited under the same conditions. We have assumed 3 that soft collisions or orbiting collisions are effective to induce vibrational relaxation in the s-tetrazine-Ar complex assisted by perturbation of the molecular vibration by the Van der Waals bound Ar atom. Dispersed fluorescence spectra are obtained with narrow band laser excitation (Atr 0.1 cm-) by means of a 1.5 meter J.Y. grating monochromator (0.24 nm/mm) at a resolution of 1 cm-. Fluorescence excitation and emission spectra were measured for the 0 , 16a 2, 6a and 6b levels of the molecular and complex species. The fluorescence excitation spectrum which reveals the 0o transition of the T-Ar complex shifted VAN DER WAALS COMPLEXES 127 -23 cm -1 with respect to the molecular transition is presented in Ref. 6. Blue shifted bands at 34 and 38 cm -1 and 43 cmfrom the T-Ar complex band are attributed to respectively the Van der Waals bending (2 x and stretching modes. Dispersed fluorescence emission spectra of the T-Ar complexes were obtained for the single vibronic levels -, la2, and -. (The complex levels and transitions are indicated by a bar above the molecular level notation.) The spectra could be well compared with the known resonance emission spectra of the parent molecule. 4 The emission spectra of the molecular species and the complex species which have to be compared, were run under the same conditions in the cold molecular jet. The results will be discussed. For each transition, the main observed emission bands are represented in Tables I-IV. The full interpretation of the results will be discussed elsewhere. 6 The main SVL emission bands of T-Ar after oo excitation are given in Table I. The main transitions from the spectra of tetrazine-argon after 16ao , and excitation are summarized in Tables II, III and IV respectively. Relative fluorescence intensities are presented in these tables for the appropriate excitation wavelengths. It is worth noticing that the relative fluorescence intensities are strongly dependent on the excitation wavelength within the broad (3 cm -, FWHM) excitation band of the particular transition within the complex.
A comparison with the resonance emission of the parent molecule shows that several vibrational levels are populated in the complex as a result of intermolecular or intramolecular vibrational relaxation.  16b The emission spectra of 6a T-Ar and T-Ar2 were published in Ref.
3. In Figure 1 we present the emission spectrum after o excitation.
The results given in Tables III and IV both for 0 as well as for excitation indicate, that as a result of vibrational relaxation within the complex, mainly the 16a , i6a11b and fir levels are formed. Vibrational predissociation, a relaxation process of the complex with ejection of the argon atom, mainly produces tetrazine in the 16a and 16b level.    After excitation in the a-ra level the emission spectrum tells us that the vibrational energy mainly flows to the 16a 2 mode of the complex and to the 16a mode of the molecular species. The first process a-ra ; 16a2 (1) must be due to vibrational redistribution; the second process due to vibrational predissociation either from the h-fa level directly i.e., fa --16a + Ar (2) or from a consecutive process of process (1) i.e., 16a 2 . 16a +Ar (la) A first indication is obtained from the graphically determined decay times for the three levels, as compared with the decaytimes of stetrazine in the same vibrational levels (Table V). In the 6a level a similar decay time is observed for excitation of the molecule and for the argon complex. This indicates that the processes (1) and (2) do not compete with the main deactivation process, i.e., the photodissociation of s-tetrazine into its products HCN and N2.
For the 16a 2 level we see a drastic reduction of the decay time indicating that the predissociation process (la) is an extra channel competing with the photodecomposition of the molecule. A similar reduction in decaytime is observed after direct excitation of the 16a vlevel of the complex. Here the predissociation channel 16a -16a +Ar (3) also reduces the decay time of the 16a level. In order to proof these interpretations we performed risetime experiments for the different levels mentioned in the processes (1) to (3). For this we used a fast cross-field photomultiplier with a risetime of 80 ps (10 to 90%). Unfortunately the decay time of this detector was not as fast as its rise-time, the cause of which is not understood yet. It will, however, not influence the decay times presented in this paper.
The growth and decay of the prepared ra level of the T-Ar complex was measured and is represented in Figure 2. The observed growth of this resonance emission equals the risetime of the detector and electronics, because the excitation was performed with a short (7 ps, FWHM) pulse from the synchronously pumped dye laser. The decay time obtained for the complex level is 520 ps, similar to that observed for the molecular species in the 6a level (Table V)_.
The experimental growth and decay curve of the formed 16a level is represented in Figure 3 (dotted curve). If we assume, according to process (1), that the 16a level is formed directly from the excited level, then the growth and decay has to be a convolution of the here that this decay time is a factor ot 2 shorter than that graphically obtained from the logarithmic plot ot the experimental curve, not taking into account the growth and decay ot the level trom which it is formed as is the case for the numbers given in Table V and ReL 3. Finally the growth and decay of the 16a level was measured and compared with the calculated curve as if it was tormed either by process (la) or by process (2). it the 16a level is formed according to the predissociation process (1a than a convolution of the pulseform ot the 16a2 emission with the appropriate 16a decay time has to give the best fit between calculated and experimental curve. If on the other hand process (2) is the correct channel for the predissociation process than the convolution has to be carried out with the pulseform of the emission. The best fit is obtained in agreement with process (la) (Figure 4). Hence the results indicate that process (2) can be neglected, which brings us to the conclusion that predissociation of the T-Ar complex does not take place directly from the prepared ra level. On the other hand vibrational predissociation is appreciable from the 16a 2 level of the complex. We might conclude that a s-tetrazine argon complex prepared in an in-plane vibrational mode first has to redistribute its vibrational energy to an out of plane 16a 2 complex mode before it predissociates to a molecular species in the 16a level and an argon atom. Hence vibrational relaxation and vibrational predissociation of a Van der Waals complex are consecutive processes and do not take place parallelly from the prepared level. From the results presented in Figure 3 we calculate a vibrational predissociation rate of 2.3 109 s -1 for the 16a level.
A similar result is obtained by kinetic studies of the 16a 2 level after direct excitation of the 16ao band. Clearly, a coupling between the out of plane 16a mode of s-tetrazine and the Van der Waals stretching mode (43 cm-x) is also illustrated by the results presented in this paper (see also Ref. 6).