(1 + 1) Resonance-enhanced multiphoton ionization spectroscopy of jet-cooled C2H2, C2HD and C2D2 in the range 46 400–48 400 cm–1

Abstract

(1 + 1) Resonance-enhanced multiphoton ionization (REMPI) spectroscopy has been used to record spectra of jet-cooled C₂H₂, C₂HD and C₂D₂ simultaneously in a time-of-flight mass spectrometer (TOF/MS) over the wavenumber range 46 400–48 400 cm^–1. The spectra for C₂H₂ and C₂D₂ are compared with previous absorption spectra recorded for room-temperature samples and laser-induced fluorescence spectra of jet-cooled molecules. For C₂D₂, the spectra are assigned beyond 47 860 cm^–1, thereby extending the known spectroscopy of this molecule. For C₂HD, assignments are suggested for the entire wavelength range recorded, and the spectrum is seen to be dominated by long progressions in the ν^′₂, ν^′₃ and ν^′₆ vibrational modes. Hot bands originating from molecules with one or two quanta of the trans- bending ν^″₄ vibrational mode are observed because of the favourable Franck–Condon factors for excitation from these vibrational levels to the à state (ù A_u for C₂H₂ and C₂D₂, ùA″ for C₂HD). To assist in the spectral assignment and remove possible ambiguities between hot bands and transitions from the zero-point vibrational level of the X¹Σ_g⁺(X ¹Σ⁺ 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 photolysis of vibrational ground state and vibrationally excited molecules. The vibrational constants obtained from fits to the REMPI spectral data agree well with the literature values. We suggest some reassignments of spectral features of C₂D₂ in light of the results of our photodissociation experiments.