Rovibrationally selected and resolved pulsed field ionization-photoelectron study of propyne: Ionization energy and spin-orbit interaction in propyne cation

Abstract

By using a high-resolution infrared (IR) laser to prepare propyne (C(3)H(4)) in selected rotational levels of the excited nu(1) (acetylenic C-H stretching) vibration mode prior to vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) measurements, we have obtained rotationally resolved VUV-PFI-PE spectra for the C(3)H(4) (+)(X (2)E(32,12),nu(1) (+)=1) band. The analysis of these PFI-PE spectra leads to the determination of the spin-orbit constant of A=-13.0+/-0.2 cm(-1) for the C(3)H(4) (+)(X (2)E(32,12),nu(1) (+)=1) state. Using this A constant and the relative rotationally selected and resolved state-to-state photoionization cross sections thus measured, we have obtained an excellent simulation for the VUV-PFI-PE origin band of C(3)H(4) (+)(X (2)E(32,12)), yielding a value of 83 619.0+/-1.0 cm(-1) (10.367 44+/-0.000 12 eV) for the adiabatic ionization energy of C(3)H(4) [IE(C(3)H(4))]. The present two-color IR-VUV-PFI-PE study has also made possible the determination of the C-H stretching frequencies nu(1) (+)=3217.1+/-0.2 cm(-1) for C(3)H(4) (+)(X (2)E(32,12)). The spectral assignment and simulation were guided by high-level ab initio calculations on the IE(C(3)H(4)), Franck-Condon factors for photoionization transitions, and rotational constants and vibrational frequencies for C(3)H(4) (+).

Photoionization spectroscopy of CH3C3N in the vacuum-ultraviolet range

“…It is of interest to mention that the measured ionization thresholds towards theX þ andB þ states of cyanopropyne cation (10.77 eV and 13.37 eV, respectively) are very close to the first ionization threshold of propyne (10.37 eV [36]) and hydrogen cyanide (13.6 eV [37]), respectively. This situation is similar to what was encountered in the case of cyanoacetylene [38] for which the ionization thresholds were close to the one of acetylene and hydrogen cyanide.…”

Ion spectroscopy: Where did it come from; where is it now; and where is it going?

“…In addition, if the IR laser excites a particular rotational state of the neutral molecule, and laser 2 is used to ionize this ro-vibrationally selected molecule, the spectrum is greatly simplified because very few rotational levels will be excited due to the predominance of the ⌬J ϭ Ϯ1 selection rule. An example of these effects is shown in Figure 2 for the case of propyne [39]. In the upper two traces of Figure 2 are shown the 1 photon VUV photoionization efficiency scan and the pulsed field ionization photoelectron spectrum of propyne.…”

“…The transition is to the vibrationally excited state of the ion, which accounts for the shift in the wavelength between the one color and two-color experiments. Figure is adapted from Xing et al [39].…”

“…By scanning the IR laser and keeping the VUV laser fixed, Xing et al [39] mapped out the neutral IR spectrum, which exhibited well resolved P, R, and Q branches in the transition to the v 1 acetylenic C-H transition at 3335.2 cm Ϫ1 . By fixing the IR laser to one of these rovibrational transitions and scanning the VUV laser through the ionic manifold of states, a single rotational level in the v level of the propyne molecule could be ionized.…”

“…The broad PFI-PES peak with a width of about 40 cm Ϫ1 (4 meV) is not resolved because of the many rotational transitions and because it consists of two electronic states that are split by spin-orbit interaction into the E 1/2 and E 3/2 states. Xing et al [39] were able to simulate the spectrum, but an ionization energy based on a fully resolved spectrum would clearly be desirable.…”