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J. Phys. Chem. 1982, 86, 56-59
Vacuum Ultraviolet Laser-Induced Photodissociation of Cyanogen Iodide. CN B2Z+ State Population and Kinetics S. Randolph Long and James P. Rellly’ Department of Ctmmlstty, Indiana University, Bloomington, Indiana 47405 (Received: August 11, 1981; In Final Form: September 2 1, 198 1)
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The single photon photodissociation_ofcyanogen iodide (ICN) is induced with radiation from a molecular fluorine laser at 1578 A. The nascent CN B2Z+population distribution, monitored by the B28+ X2Z+emission, is characterized by high rotational and moderate vibrational excitation. Rotational distrib_utionsappear roughly Boltzmann with Trot= -14000, 6000, 3000, and 1800 K for u’ = 0, 1, 2, and 3 of the B state. Photolysis of ICN/Ar mixtures leads to B state u’ = 0 emission at times long after radiative depletion of the nascent B state popdation. Both spzctral and time-resolved studies demonstrate that collision-induced transfer from u ’ = 10 of A 2 n to u’ = 0 of B28+is responsible for the long-time B X emission.
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Introduction The molecular fluorine laser’ at 1578 A provides photochemists with a superb source of vacuum ultraviolet radiation. Its significance derives from its wavelength, which is near the Suprasil quartz cutoff, its high monochromaticity, and its short pulse duration. It should be possible to perform useful photochemical experiments with this light source which complement those previously undertaken with flash lamps or atomic resonance line emission. Unfortunately the lack of tunability limits its utility to the study of diffuse vacuum UV continua or fortuitous molecular coincidences. But, as we demonstrate in this experjment on cyanogen iodide (ICN), in such cases this laser light source can indeed lead to new high-resolution photochemical data not extractable with conventional techniques. In this study y e exambe wavelength- and time-resolved spectra of CN B22+ X22+fluorescence following photodissociation of ICN. This system has been the subject of several previous investigations, and it is well-known that cyanogen radicals are produced in both the A and B excited electronic states with considerable vibrational and rotational e x c i t a t i ~ n . ~While -~ our initial goal was to obtain a complete product state distribution for B22+to be used in a surgrisal analysis of the dissociative channel leading to the B state, the_exten_siveblending and overlapping of X fluorescence spectrum largely lines in the CN B precluded this. We do report, however, vibrational-level popgations and approximate rotational temperatures for the B state levels produced in the photodissociation. More importantly, we exploit the time resolution attainable with the F2 laser in examining kinetic processes resulting from collisions between CN* and argon. In thest observations, the existence of perturbations between the A21Tiand B28+ states”l0 is made particularly evident through observations
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with both temporal and spectral resolution.
Experimental Section The laser employed in these studies is a Lumonics Model TE-861 excimer laser operating on the molecular fluorine transition at 1578 A. The manufacturer’s specifications stipulate a pulse energy of about 12 mJ. The temporal width of the laser pulse is approximately 15 ns, and its spectral width is
