(1-ENHANCED MULTIPHOTON IONIZATION SPECTROSCOPY OF JET-COOLED C2H2, C2HD AND C2D2 IN THE RANGE 46400-48400 CM(-1)(1) RESONANCE)

(1+1) Resonance-enhanced multiphoton ionization (REMPI) spectroscopy h as been used to record spectra of jet-cooled C2H2, C2HD and C2D2 simul taneously in a time-of-flight mass spectrometer (TOF/MS) over the wave number range 46400-48400 cm(-1). The spectra for C2H2 and C2D2 are com pared with previous absorption spectra recorded for room-temperature s amples and laser-induced fluorescence spectra of let-cooled molecules. For C2D2, the spectra are assigned beyond 47 860 cm(-1), thereby exte nding the known spectroscopy of this molecule. For C2HD, assignments a re suggested for the entire wavelength range recorded, and the spectru m is seen to be dominated by long progressions in the nu'(2), v'(3) an d v'(6) vibrational modes. Hot bands originating from molecules with o ne or two quanta of the trans-bending nu''(4) vibrational mode are obs erved because of the favourable Franck-Condon factors for excitation f rom these vibrational levels to the (A) over tilde state ((A) over til de(1)A(u), for C2H2 and C2D2, (A) over tilde(1)A'' for C2HD). To assis t in the spectral assignment and remove possible ambiguities between h ot bands and transitions from the zero-point vibrational level of the (X) over tilde(1) Sigma(g)(+) ((X) over tilde(1) Sigma(g)(+) for C2HD) electronic state, we carried out photolysis experiments on jet-cooled C2HD and C2D2. The photodissociation is performed by a laser tuned to different vibrational features of the REMPI spectrum, and the kinetic energies of the resulting hydrogen or deuterium atoms are determined by TOF measurements. This technique distinguishes clearly between phot olysis of vibrational ground state and vibrationally excited molecules . The vibrational constants obtained from fits to the REMPI spectral d ata agree well with the literature values. We suggest some reassignmen ts of spectral features of C2D2 in light of the results of our photodi ssociation experiments.