State-selected and state-to-state photoionization-photoelectron spectroscopy using two-color infrared and vacuum ultraviolet lasers

Two-color infrared (IR) and vacuum ultraviolet (VUV) lasers are employed to study the photoionization dynamics and photoelectron spectroscopy of polyatomic molecules. Using the IR-VUV scheme together with the pulsed field ionization-photoelectron (PFI-PE) technique, we have demonstrated that the preparation of the neutral molecule in a well-defined single rovibrational state by the IR laser excitation prior to VUV laser PFI-PE measurement can greatly simplify the photoelectron spectrum, allowing unambiguous rotational assignment and determination of ionization energies with significantly improved precision. In addition, with IR excitation, the Franck-Condon factors (FCF) are mediated such that new transitions not favorable in single-photon experiments can be probed. This technique provides a unique way to study state-selected and state-to-state photoionization dynamics of polyatomic molecules. In the VUV-IR scheme, high Rydberg states are first excited by the VUV laser, followed by an IR excitation on the ion core. This technique enables us to examine the vibrational bands involving excitation of C-H, O-H, and N-H stretching modes of the cation, which are usually unobservable in VUV photoionization because of unfavorable FCFs or restricted selection rules. Thus the IR-VUV and VUV-IR techniques are complementary to the single photon VUV photoionization schemes. Compared to previously reported IR-UV-UV or UV-UV-IR photoionization experiments, the IR-VUV and VUV-IR schemes do not require the existence of long-lived, bound intermediate rovibronic states in the UV region and are thus more general methods. We expect that the IR-VUV scheme will have broad applicability to state-selected and state-to-state photoionization studies of polyatomic molecules and will become a valuable spectroscopic technique for the study of polyatomic ions.