Optical probes of atomic and molecular decay processes.
- Chemistry
The study of molecular photoionization and photodissociation dynamics provides insight into the intramolecular mechanisms by which energy and angular momentum are exchanged and redistributed among the internal degrees of freedom of highly excited molecules and, more specifically, into the mechanisms that determine the decay pathways and resulting product-state distributions for the excited molecules. These mechanisms lie at the heart of one of the principal subjects of chemistry, that is, understanding and controlling the factors that govern the making and breaking of chemical bonds. The objective of this experimental research program is to elucidate these fundamental mechanisms and to provide useful prototypes for the development of a general qualitative understanding of their ramifications. In this program, the primary focus is on resonant processes in the ionization and dissociation continua, that is, on autoionization and predissociation. These processes are studied as a function of the electronic, vibrational, and rotational quantum numbers of the resonances, allowing a better understanding of their fundamental mechanisms. In the past three years, the primary emphasis of this experimental program has been on understanding the process of vibrational autoionization in Rydberg states of small polyatomic molecules. Vibrational autoionization corresponds to the decay of resonances above the ionization threshold into the continuum through the conversion of vibrational energy into electronic/translational energy of the highly excited/ejected electron. In polyatomic molecules, I am particularly interested in determining how this process depends on both the specific normal vibrational modes involved in the process and the electronic character of the resonances. In this program, the experimental approach relies on laser-based, double-resonance techniques to prepare the selected excited states in the molecules of interest, and on a variety of detection techniques to characterize both the photoexcitation and subsequent decay processes. These techniques include mass spectrometry, dispersive and threshold photoelectron spectroscopy, laser-induced fluorescence, fluorescence-dip spectroscopy, and laser-induced grating spectroscopy. While the instrumentation is currently available for each of these techniques, two instruments deserve special mention. First, a high-resolution magnetic-bottle electron spectrometer has been developed that is equipped with a pulsed, skimmed molecular beam source. This instrument is capable of {approx}3-4 meV resolution in the electron kinetic energy while providing a collection efficiency of {approx}50%. Second, a time-of-flight mass spectrometer has been constructed with a similar molecular beam source. This instrument is currently being adapted to allow both ion- and electron-imaging studies. A typical experimental study is performed in three steps. First, resonant one-color multiphoton ionization is used to map out the transition between the ground state of the molecule of interest and the low-lying excited state to be used as an intermediate in the double-resonance process. The lasers used in these studies are Nd:YAG-pumped dye lasers with {approx}5 ns pulse durations. In general, this pump transition corresponds to a one- or two-photon process, and the laser output is frequency doubled, tripled, or mixed to generate light in the region of interest. In molecules such as ammonia and aniline, the pump transitions of interest are well characterized, allowing the unambiguous choice of pump transitions that access levels with the rovibronic character of interest. In other cases, the spectroscopy of the pump transition must be analyzed before it can be useful for the double-resonance experiments. In the second step, the pump laser is fixed on the pump transition of interest and a second laser is used to probe transitions from the upper state of the pump transition to the autoionizing or predissociating resonances in the region of interest. The double-resonance spectrum is recorded by scanning the second laser and monitoring either the mass-selected ion signal or the total photoelectron signal resulting from the two-color process.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 1013963
- Report Number(s):
- ANL/CHM/CP-118502; TRN: US201111%%2
- Resource Relation:
- Conference: 27th Annual Combustion Research Meeting; May 30, 2006 - Jun. 2, 2006; Wintergreen, VA
- Country of Publication:
- United States
- Language:
- ENGLISH
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