We have studied 3s(n-1 and pi-1) Rydberg states and D0(n-1) and D1(pi-1) cationic states of pyrazine [1,4-diazabenzene] by picosecond (2 + 1) resonance-enhanced multiphoton ionization (REMPI), (2 + 1) REMPI photoelectron imaging, He(I) ultraviolet photoelectron spectroscopy (UPS), and vacuum ultraviolet pulsed field ionization photoelectron spectroscopy (VUV-PFI-PE). The new He(I) photoelectron spectrum of pyrazine in a supersonic jet revealed a considerably finer vibrational structure than a previous photoelectron spectrum of pyrazine vapor. We performed Franck-Condon analysis on the observed photoelectron and REMPI spectra in combination with ab initio density functional theory and molecular orbital calculations to determine the equilibrium geometries in the D0 and 3s(n-1) states. The equilibrium geometries were found to differ slightly between the D0 and 3s states, indicating the influence of a Rydberg electron on the molecular structure. The locations of the D1-D0 and 3s(pi-1)-3s(n-1) conical intersections were estimated. From the line width in the D1 <-- S0 spectrum, we estimated the lifetime of D1 to be 12 fs for pyrazine and 15 fs for fully deuterated pyrazine. A similar lifetime was estimated for the 3s(pi-1) state of pyrazine by REMPI spectroscopy. The vibrational feature of D1 observed in the VUV-PFI-PE measurement differed dramatically from that in the UPS spectrum, which suggests that the high-n Rydberg (ZEKE) states converging to the D1 vibronic state are short-lived due to electronic autoionization to the D0 continuum.

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) (+).

The vacuum ultraviolet (VUV) pulsed field ionization−photoelectron (PFI−PE) spectrum for trans-dichloroethene (trans-ClCHCHCl) has been measured in the energy range 77 600−79 200 cm-1. On the basis of the spectral simulation of the origin VUV−PFI−PE vibrational band, we have determined the IE(trans-ClCHCHCl) to be 77 678.4 ± 2.0 cm-1 (9.630 97 ± 0.000 25 eV). The vibrational bands resolved in the VUV−PFI−PE spectrum of trans-ClCHCHCl are assigned on the basis of ab initio vibrational frequencies and calculated Franck−Condon factors for the ionization transitions, yielding eight vibrational frequencies ν1 + = 163 cm-1, ν3 + = 367 cm-1, ν4 + = 871 cm-1, ν5 + = 915 cm-1, ν6 + = 944 cm-1, ν8 + = 1235 cm-1, ν9 + = 1258 cm-1, ν10 + = 1452 cm-1. The distinct feature of the VUV−PFI−PE spectrum is the strong vibrational progression of the ν3 + (CCl stretching) mode of trans-ClCHCHCl+, which is consistent with the theoretical geometry calculation, predicting a significant change in the CCl bond distance upon photoionization of trans-ClCHCHCl. We have also determined the frequency (3068 cm-1) for the ν11 + (CH stretching) vibrational mode of trans-ClCHCHCl+ by employing the VUV−IR-photoinduced Rydberg ionization (VUV−IR−PIRI) method. The VUV−IR−PIRI spectra for trans-ClCHCHCl prepared in the effective principal quantum numbers, n* = 14 and 36, are found to be identical, supporting the previous conclusion that the Rydberg electron behaves as a spectator; i.e., the Rydberg electron orbital is conserved during the IR excitation of the ion core.

We show that by preparing ethylene (C2H4) in selected rotational levels of the C−H stretching v 9(b2u) = 1 or v 11(b1u) = 1 state by infrared (IR) laser excitation prior to vacuum ultraviolet (VUV) laser photoionization, greatly simplified, rotationally resolved pulsed field ionization-photoelectron spectra can be obtained, leading to reliable rotational assignments of the vibrational bands for C2H4 +(X̃2B3u; v 4 +(torsional) = 1 or v 11 + = 1) ← C2H4(X̃Ag; v 11 = 1) and C2H4 +(X̃B3u; 00+) ← C2H4(X̃1Ag; v 9 = 1). The observation of these Franck−Condon (FC) forbidden vibrational bands is indicative of a strong state-mixing mechanism, in which Rydberg states converging to the FC forbidden C2H4 +(X̃2B3u; 00+ and v 4 + = 1) levels are effectively coupled via intermediate states to Rydberg levels converging to FC allowed C2H4 +(X̃2B3u; v 9 + = 1 and v 11 + = 1) states, respectively. This conclusion is supported by strong autoionizing features observed in the IR−VUV photionization efficiency spectra.

We have observed fully rotationally resolved transitions of the photoelectron vibrational bands 2(4), 2(5), 1(1)2(1), and 1(1)2(3) for ammonia cation (NH3+) by two-color infrared (IR)-vacuum ultraviolet (VUV)- pulsed field-ionization photoelectron (PFI-PE) measurements. By preparing an intermediate rovibrational state of neutral NH(3) with a known parity by IR excitation followed by VUV-PFI-PE measurements, we show that the photoelectron parity can be determined unambiguously. The IR-VUV-PFI-PE measurement of the 2(4) band clearly reveals the formation of both even and odd l states for the photoelectrons, where l is the orbital angular momentum quantum number. This observation is consistent with the conclusion that the lack of inversion symmetry for NH3 and NH3+ allows odd/even l mixings, rendering the production of both odd and even l states for the photoelectrons. Evidence is also found, indicating that the photoionization transitions with DeltaK=0 are strongly favored compared to that with DeltaK=3. For the 2(5), 1(1)2(1), and 1(1)2(3) bands, only DeltaK=0 transitions for the production of even l photoelectron states from the J'K'=2(0) rotational level of NH3(nu1=1) are observed. The preferential formation of even l photoelectron states for these vibrational bands is attributed to the fact that the DeltaK=0 transitions for the formation of odd l photoelectron states from the 2(0) rotational level of NH3(nu1=1) are suppressed by the constraint of nuclear-spin statistics. In addition to information obtained on the photoionization dynamics of NH3, this experiment also provides a more precise value of 3232+/-10 cm-1 for the nu1+ (N-H stretch) vibrational frequency of NH3+.