Double-resonance spectroscopy of autoionizing states of water

The goal of this research program is to develop a conceptual understanding of the mechanisms for the conversion of energy and angular momentum among the electronic, vibrational, rotational, and translational degrees of freedom in the highly excited states of a prototypical triatomic molecule, water. Such microscopic mechanisms serve as prototypes for the mechanisms responsible for elementary chemical reactions. The goal of this program will be addressed by studying the spectroscopy and dynamics of the Rydberg states of water near their first ionization thresholds; these states are particularly interesting because they can decay by either ionization or dissociation. The dependence of these competing decay processes on the electronic, vibrational, and rotational quantum numbers of both the decaying state and its fragments is expected to provide substantial insight into the mechanisms of both processes. The experiments will be performed by using a double resonance scheme using VUV light to pump an intermediate Rydberg state; subsequent absorption of light from a second laser will prepare highly excited Rydberg states in a state-selective manner, and the different decay products will be monitored by using a variety of detection techniques.

Early work on this project has yielded the first double resonance spectra of vibrationally autoionizing states of water. Previous work utilized one-photon excitation directly from the ground state; as a result, final states were excited unselectively from several ground rotational states, and the observed Rydberg autoionizing state transitions were weak, complicated, and rode on a large background of direct photoionization. This made comparison to theory somewhat speculative. Use of intermediate resonance with a relatively low-lying Rydberg state offers a much cleaner experiment on several counts. First, a single rotational state of the intermediate state can be populated, resulting in drastic spectral simplification and a labeling of the possible angular momenta of the final states. Second, since a Rydberg state has nearly the same geometry as the molecular ion's ground state, one can excite particular final vibrational states with a high degree of selectivity by choosing the same vibrational state in the intermediate state (thank you, Franck and Condon).

Figure 1 shows the excitation scheme schematically. The intermediate state used for this study is the C ¹B₁ state with one quantum of symmetric stretch (100). The tunable VUV light near 120 nm which drives the first step is produced using two-photon resonant difference frequency generation in krypton gas. This process yields high conversion efficiency so that a high enough intensity can be produced to excite a large fraction of the ground state molecules to the C state. Strong excitation is required on the first step due to the rapid predissociation of the C state, which results in a large loss of the excited state population before it can be further excited to the high Rydberg states of interest. A secon laser tunable between 550 nm and 470 nm promotes the excited molecules to the region between the ionization limit and the threshold for production of ions in the (100) vibrational state. In this region lies our quarry, Rydberg states with one quantum of symmetric stretch which are degenerate with, and coupled to, the continuum of the (000) vibrationally unexcited states. The vibrationally excited states autoionize due to their coupling with the continuum, leading to resonances in the photoionization probability versus wavelength.

Figure 2 shows a spectrum of autoionizing states in the region of principal quantum number of n=6, excited from the 1₁₁ rotational state. The (100) states stand out strongly above a small direct photoionization background, and the signal-to-noise is pretty nifty compared to previous work. Detailed analysis of such spectra are underway in collaboration with Mark Child at Oxford. We now know that, surprisingly, the sharp transitions in the spectrum are due to excitation of nf states, the first time such states of water have been observed. The broader peaks are nd state transitions.

Just to show that we can see them, figure 3 shows a spectrum of J=1 high-n vibrationally autoionizing Rydberg states converging to several rotational levels of the ion core, excited from the 0₀₀ rotational state. This is the first time these states have been observed, and should provide a fascinating test of quantum defect theory for this important triatomic molecule.

Last Updated: July 9 1997 Webmaster