Appl. Phys. B 32, 35-38 (1983) Applied ""'- Physics B "" 9 Springer-Verlag 1983 Resonance-Enhanced Multiphoton Ionization (2 + 1) of the/} and C' States of Ammonia R. J. Stanley, O. Echt*, and A. W. Castleman, Jr. The Pennsylvania State University 152 Davey Laboratory, Department of Chemistry, University Park, PA 16802, USA Received 29 April 1983/Accepted 10 May 1983 Abstract. Resonance-enhanced nmltiphoton ionization (REMPI) of the /~ and C' states of NH 3 in a pulsed supersonic molecular beam was observed at 30~340nm, using the multiphoton-ionization mass-spectrometer system described herein. The observed features are the result ofa 2 + 1 MPI process and correspond quite well with the reported 3 + 1 MPI of NH 3 in the literature from the: ground state of opposite parity. PACS: 33.80.Kn ; 33.80.Eh ; 33.20.Lg The study of resonance-enhanced multiphoton ioniza- tion (REMPI) spectroscopy of ammonia has been pursued extensively by Colson and coworkers [1-5]. During the course of their work, they discovered a new state [1, 2] via (3+ 1)multiphoton ionization. This (~' state is isoenergetic with the C state but has never been observed in conventional spectroscopic studies in the ultraviolet. Rotational analysis of this state was found to favorably compare to a calculated rotational distri- bution based on a theory for three photon absorption by Nieman [2], thereby further supporting the identifi- cation. On the basis of the observed parallel band structure, they assigned its symmetry as 1A' 1. Using similar techniques, these researchers have also in- vestigated the REMPI (3 + 1) spectra of the/} state. The phenomenon presented herein was observed while working with the third harmonic of the Nd:YAG laser. At 355 nm, a strong MPt signal for NH 3 and its clusters was observed for laser powers >3m J/pulse. The signal was found to be the result of a 2 + 1 process which has hereto fore not been reported [6] for NH 3. Three 355 nm photons correspond to 10.5 eV, which is 0.34 eV above the ionization potential for the ammonia molecule. A 2+1 REMPI process to the /} and C' states, analogous to the 3 + 1 process, should exist in * NATO Fellow, 1982/83. Permanent address: University Konstanz, Fakult~it fiir Physik, D-7750 Konstanz, Fed. Rep. Germany this energy range. However, a change in selection rules should require it to originate from the ground-state vibronic level of opposite parity. Spectra were taken over the range 304 340 nm in order to search for the expected resonances. The results of these studies are reported herein. Experimental The molecular-beam multiphoton-ionization mass- spectrometer employed in the present experiments is shown schematically in Fig. la and b, The apparatus consists of two chambers, a beam chamber (I) which houses the molecular-beam source, and a detection chamber (II) where the molecular beam is intersected at right angles by the laser beam between plates 1 and 2 in Chamber II. The resulting ions are extracted from the interaction region and focused into a quad- rupole mass-spectrometer where they are mass selec- ted. The ions then strike a high-gain particle multiplier and the resulting amplified ion current is further amplified as needed, and sent to a boxcar integrator. This device may be set up as a straight integrator as well. The output of the integrator is digitized and stored on floppy disk with help of a dedicated micro- computer (Fig. lb). All signal acquisition averaging operations are perfor- med via microcomputer and the time constant of integration is set at 2-10gs for most work. The