The infrared-ultraviolet dispersed fluorescence spectrum of acetylene: Newclasses of bright states

Single rotational levels of ungerade vibrational levels, 2 nu (3)'+nu (6)' and 3 nu (3)'+nu (6)' (both with b(u) symmetry), in the (A) over tilde (1)A (u) electronically excited state of acetylene were excited by an IR-UV doub le resonance scheme via the nu (3)" fundamental level in the (X) over tilde (1)Sigma (+)(g) state, and the rotationally resolved dispersed fluorescenc e (DF) spectra were recorded at 3.2-4.5 cm(-1) resolution. The term values of the new ungerade levels were determined within an accuracy of 0.56 cm(-1 )(1 sigma) through careful calibration achieved by frequency standard atomi c Fe and Hg lines. A total of 111 new ungerade vibrational levels with Sigm a (+)(u), Sigma (-)(u), and Delta (u) symmetry below 10 000 cm(-1) was iden tified in the high-resolution IR-UV-DF spectra, which provide access to new classes of (X) over tilde (1)Sigma (+)(g) bright states: (i) (0,upsilon (2 )" ,0,upsilon (4)" (1),1(-1))Sigma (+)(u), (0,upsilon (2)" ,0,upsilon (4)" (1),1(1))Delta (u), and (0,upsilon (2)" ,0,upsilon (4)" (3),1(-1))Delta (u) , which are the Franck-Condon (FC) bright levels from the n nu (3)' + nu (6 )' (n=2,3) levels in the (A) over tilde (1)A(u) state; (ii) (0,upsilon (2)" ,0,upsilon (4)" (-1),1(1))Sigma (-)(u) levels which appear through the a-a xis Corioris interaction between n nu (3)' + nu (6)' and n nu (3)' + nu (4) ' (n=2,3) in the A (1)A(u) state; and (iii) (0,upsilon (2)" ,1,upsilon (4)" (0),0)Sigma (+)(u) and (0,upsilon (2)" ,1,upsilon (4)" (2),0)Delta (u) lev els which gain transition intensity from the Duschinsky effect associated w ith the bent-linear (A) over tilde-(X) over tilde transition. All observed ungerade term values and previously determined gerade and ungerade term val ues below 10 000 cm(-1) were fitted by two effective model Hamiltonians, i. e., a pure-bend effective Hamiltonian and a stretch-bend effective Hamilton ian. The stretch-bend effective Hamiltonian is expressed in terms of 31 Dun ham expansion parameters and 11 anharmonic resonance parameters associated with (i) five stretch-bend anharmonic resonances; (ii) one stretch-stretch and two bend-bend Darling-Dennison resonances; and (iii) one vibrational l resonance. The parameters in this Hamiltonian were determined from a least- squares fit of 287 vibrational term values (111 new ungerade levels, 128 le vels from absorption, 1 level from stimulated Raman, 13 levels from stimula ted emission pumping (SEP), and 34 levels from UV-DF spectroscopy) below 10 000 cm(-1) with a standard deviation of sigma = 1.21 cm(-1). The FC patter ns for the v(4)" = odd ungerade levels, (0,v(2)" ,0,v(4)" ,1), in the IR-UV -DF spectra were derived, and the nodes along the v(4)' trans-bend mode wer e found at v(4)" = 11 via the 2 nu (3)' + nu (6)' upper state, and at v(4)" = 9 and 15 via the 3 nu (3)' + nu (6)' upper state, which is consistent wi th the nu (')(3) dependence of the FC patterns observed in previous UV-DF s tudies. (C) 2001 American Institute of Physics.