Action Spectroscopy and Predissociation of Vibrationally Excited C₂HD

Abstract

H and D photofragments, produced via vibrationally mediated photodissociation of jet-cooled C2HD allowed measurement of vibrational and vibronic excitation action spectra of the parent as well as H and D Doppler profiles. The initial vibrational preparation at energies around 9700 cm−1 enhanced C—H and C—D bond cleavage for transitions from an initially excited bent state to the upper A ~ state, rather than from a close lying state containing three C—H stretch quanta, due to favorable Franck—Condon factors for the former. The bound region of the A ~ state was accessed, resulting in non-adiabatic dissociation and slight preferential C—H over C—D bond cleavage.

Vibrationally mediated photodissociation of ethyne isotopologues and homologues revisited

“…The transitions to this dark state are probably characterized by low transition dipole moment for vibrational excitation and by relatively large FC factor for the subsequent UV excitation to the upper state. A similar behaviour was encountered in ethyne and mono-deuterated ethyne, where combination states including bending were preferentially enhanced relative to C-H stretch overtones and consequently showed up preferentially in the action spectra [4,5,9,11,12,14,15].…”

“…Bar and S. Rosenwaks spectra, Boltzmann distribution at the involved temperatures, and a Voigt line shape. The weak absorption to the (2100 0 2 0 ) bent state (exemplified both in the simulation, Figure 5(b) and in the PA spectrum [15]) clearly points to enhancement in the photodissociation from this initially prepared state as compared to that from (3000 0 0 0 ). Furthermore, the couple of transitions accessing J 0 ' ¼ 11 of (3000 0 0 0 ) and to a lower extent the couple of transitions to J 00 ¼ 6, 7, 8 of (2100 0 2 0 ) deviate from the rotational contour, meaning that a deviation from the Boltzmann distribution is encountered.…”

“…C 2 HD in the 3 1 and 2 1 þ 2 þ 2 5 regions [7,15] The action spectrum of D fragments, ensuing from 243.069 nm photolysis/probing (on the resonance of the two-photon D transition) of rovibrationally excited C 2 HD in the energy regions of the (3000 0 0 0 ) and (2100 0 2 0 ) bands, was monitored. Of much surprise is the resulting D action spectrum, presented in Figure 5(a), showing rovibrational transitions differing from those in the H action spectrum [15]. The observed transitions were assigned by comparing the D action spectrum to the calculated one, Figure 5(b), exposing the great difference between the two.…”

“…In the action spectrum two dominating lines, matching the J 00 (11) (P and R transitions) of the (3000 0 0 0 ) band and the P-and R-transitions corresponding to C 2 HD (2100 0 2 0 ) are observed. The calculated spectrum, Figure 5(b), accounts for the rotational constants of the (3000 0 0 0 ) and (2100 0 2 0 ) states [29,36] that were used for the PA spectrum calculation [15], for the relative intensities ((2100 0 2 0 )/(3000 0 0 0 ) ¼ (1)/(8.6)) measured in the PA…”

“…VMP of the ethyne isotopologues C 2 H 2 [2][3][4][5][6][7][8][9][10][11][12][13] and C 2 HD [6,7,[13][14][15] revealed mode-and bond-selective cleavage of the C-H or C-D bond when the molecules were preexcited to vibrational states in the ground electronic state at energies around that of three, four or five C-H stretches. On the other hand, VMP of three ethyne homologues: (normal) propyne (H 3 CCCH) [16][17][18][19][20], propyne-d 3 (D 3 CCCH) [7,21,22] and 1-butyne (H 3 CH 2 CCCH) [12,19,[23][24][25], pre-excited to energies of up to five C-H acetylenic or alkylic vibrational stretchings, did not show mode-specific cleavage but, nevertheless, in some cases the energy flow out of the pre-excited states indicated mode-dependent coupling with close-lying bath states.…”