Disclosing rovibrational couplings and overlaps from irregularities in action spectra: Photodissociation of the 4νCH rovibrational manifold of C2H2

Abstract

Absorption and free-jet action spectroscopy of states in the region of four quanta of C-H stretching of acetylene (C 2 H 2 ) around 12 676 cm Ϫ1 were studied. Rovibrational excitation combined with 243.135 nm promotion of C 2 H 2 molecules to the upper electronic trans-bent states, Ã 1 A u /B 1 B u , and H photofragment ionization generated action spectra measuring the H yield as a function of the excitation wavelength. The findings show that the (1030 0 0 0 ) IR bright state of the third C-H stretch overtone has a smaller photodissociation cross section than the (1214 0 0 0 ) combination band containing trans-bend mode excitation and lying in its vicinity, due to a favorable Franck-Condon factor for the latter. Certain line pairs accessing similar JЈ levels of the (1030 0 0 0 ) state show anomalous intensities in the action spectrum, with P transitions excessively enhanced over R transitions. This implies enormous dissociation efficacy as a consequence of Coriolis-type local resonances with nearby states of another band and due to the overlap of the P transitions with another band.

Vibrationally mediated photodissociation of ethyne isotopologues and homologues revisited

“…, as compared to those of the analogous transitions of (1030 0 0 0 ) [9]. Accordingly, the very irregular intensity pattern in the action spectra implies a very unusual behaviour in this spectral window.…”

“…The extracted relative photolysis cross sections for different rotational states of the 1030 0 0 0 state increase gradually with J 0 , approaching extremely high values as J 0 increases, in particular the R(17), P(16) and P (19) transitions. For the R(17) and P (19) lines, an additional observation is that of the doubling of these lines [9]. The line doubling is due to a local perturbation affecting the J ¼ 18 level [32,34,35].…”

“…C 2 H 2 in the 4 CH region [2,9] The room temperature PA spectrum of C 2 H 2 presented in panel (a) of Figure 4 discloses the most prominent bands signifying ethyne excitation in the 12,620-12,720 cm À1 energetic window. It is clearly seen that one of the bands, corresponding to transitions from the vibrational ground state to the (1030 0 0 0 ) state, carries most of the intensity.…”

“…The VMP of C 2 H 2 was studied for vibrational preexcitation at energies around that of the 1 CH [10] fundamental, and the 3 CH [8,11,12], 4 CH [2,9] and 5 CH [3][4][5][6][7] overtone stretchings. VMP of C 2 H 2 produces H atoms þ C 2 H radicals, while VMP of C 2 HD offers two distinct isotopic channels, H þ C 2 D and D þ C 2 H. To study these channels C 2 HD was excited, alternatively, to three stretches of CH (3 1 ) [7,5], to two stretches of CH plus one of CC and two CCH bends (2 1 þ 2 þ 2 5 ) [5,7], to four stretches of CH and one of CD (4 1 þ 3 ) [6,7], three stretches of CH plus one of Figure 1.…”

“…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.…”