An important part of our work is the development of new techniques for the study of chemical dynamics at surfaces. In this period we have had success in developing several new techniques with important implications for future work by us and others working in the field.
Production of a beam of highly vibrationally excited CO using perturbations
Nils Bartels, Tim Schäfer, Jens Hühnert, R.W. Field and Alec M. Wodtke (accepted in J. Chem. Phys.)
Figure 1: P3D concept for CO in a potential energy diagram. PUMP1 (λ1) excites to ν=0 of the metastable a3Π state (black arrow). From there PUMP2 (λ2) excites to e3Σ- (ν=12) (red arrow) which is perturbed by the A1Π1 (ν=8) state. The perturbation leads efficiently to very high vibrational levels of Χ1Σ+ (ν»0) and emission can be enhanced in a DUMP (λ3) step (green arrow).[less]
Figure 1: P3D concept for CO in a potential energy diagram. PUMP1 (λ1) excites to ν=0 of the metastable a3Π state (black arrow). From there PUMP2 (λ2) excites to e3Σ- (ν=12) (red arrow) which is perturbed by the A1Π1 (ν=8) state. The perturbation leads efficiently to very high vibrational levels of Χ1Σ+ (ν»0) and emission can be enhanced in a DUMP (λ3) step (green arrow).
The application of state-to-state scattering methods is so far only possible for a very small number of molecules. Indeed, much of the work to date on vibrational energy transfer in mo-lecular collisions with surfaces has involved NO scattering. This is because the spectroscopy of NO is very favorable both for state specific optical detection of NO and for optical pumping to prepare NO in specific quantum states. An important priority for future work is to explore the generality of the results obtained so far by doing similar experiments on other molecules.
We have developed and demonstrated a new optical pumping method that is applicable to CO. We were able to produce an intense molecular beam of CO (X1Σ+) in high vibrational states (v = 17, 18) by a new approach that we call PUMP-PUMP-PERTURB and DUMP (P3D). The basic idea is to access high vibrational states of CO e3Σ- via a two-photon doubly resonant transition that is perturbed by the A1Π state. DUMP-ing from this mixed (predominantly triplet) state allows access to high vibrational levels of CO (X1Σ+). Importantly, this approach avoids the use of vacuum UV radiation in any of the excitation steps. We demonstrated the approach by measuring the excitation of molecules in the beam by laser induced fluorescence (LIF) and resonance enhanced multi-photon ionization (REMPI) spectroscopy.
This is only the second molecule where optical pumping methods have become available for preparing it in high vibrational states with sufficient efficiency that surface scattering experiments will be possible. Such studies will be pursued in the near future and promise to dramatically broaden the available data that can be compared to first principles theory for interactions of highly vibrationally excited molecules with surfaces.
Orienting polar molecules without hexapoles: Optical state selection with adiabatic orientation
Tim Schäfer, Nils Bartels, Nils Hocke, Xueming Yang, Alec M. Wodtke, Frontier Article in Chemical Physics Letters 535 (2012) 1–11
Figure 2: The orientation electrode is mounted 1cm in front of the surface, capable to produce fields up to 20 kV/cm. The molecule is prepared in a specific parity level using high resolution lasers. The parity level adiabatically orients as it travels into the strong electric field.[less]
Figure 2: The orientation electrode is mounted 1cm in front of the surface, capable to produce fields up to 20 kV/cm. The molecule is prepared in a specific parity level using high resolution lasers. The parity level adiabatically orients as it travels into the strong electric field.
The spatial configuration of atoms serves as well as any as a definition for molecular identity, structure and, to a large extent, function. In direct analogy, how a molecule's atoms are spatially configured with respect to its surroundings helps to define its functional interactions with its environment. This statement helps explain the long standing motivation to develop means of controlling molecular orientation; that is to say: controlling how a molecule points in real space.
Nowhere has the desire to understand orientational impacts on function driven scientific investigation more than in the field of surface chemical dynamics. Specific examples that have attracted attention include orientational influences on: rotational inelastic energy transfer in molecule-surface collision, molecular sticking at surfaces and electronically nonadiabatic mole-cule surface interaction.
Recently, a new ab initio theory of electronically nonadiabatic molecule-surface interactions has predicted a strong steric effect for the vibrational relaxation of highly vibrationally excited NO. Specifically, experiments from our laboratory employing un-oriented NO(v ≫0) molecules in collisions with Au(111) reported multiquantum vibrational relaxation mediated by electron transfer. The ab initio theory predicted the energy transfer (and hence the electron transfer) is completely suppressed for O-end collisions. In order to test the assumptions underlying this highly promising theory, we were led to consider means for producing oriented samples of NO in high vibrational states using stimulated emission pumping with variable kinetic energies between about 30 and 1000 meV. In the course of designing an orientation experiment to study this phenomenon, we were confronted with some of the limitations of established methods for orientation.
This led us to develop a new approach to orientation, which has now appeared as a frontier (cover) article in Chemical Physics Letters. In this new approach laboratory frame orientation of polar molecules is achieved by state-specific optical pumping in a region free of electric fields followed by adiabatic transport into a static electric field. This approach overcomes some of the limitations of the more common hexapole focusing method. In particular the method is nearly insensitive to the kinetic energy of the sample. We demonstrate production of oriented samples of NO (μel = 0.15 D) with translational energies above 1 eV in both high- and low-field seeking states. The method can be extended to many other classes of molecules, including near symmetric tops and ions.
Generation of tunable narrow bandwidth nanosecond pulses in the deep-ultraviolet for efficient optical pumping and high resolution spectroscopy
Luis Velarde, Daniel P. Engelhart, Daniel Matsiev, Jerry LaRue, Daniel Auerbach, and Alec M. Wodtke, Rev. Sci. Instrum. 81, 063106 (2010)
Figure 3: A recently developed FT-UV light source capable of producing ns pulses with 150 MHz linewidth at 206 nm, and pulse energies greater than 4 mJ, an example of the kind of laser development project that is now routinely carried out in the Department.[less]
Figure 3: A recently developed FT-UV light source capable of producing ns pulses with 150 MHz linewidth at 206 nm, and pulse energies greater than 4 mJ, an example of the kind of laser development project that is now routinely carried out in the Department.
For optical pumping methods like those used in many of our experiments, laser light sources with high pulse energies and Fourier Transform limited bandwidth are optimal. Part of the department is now involved with building, testing and implementing novel light sources optimized for our needs. In this work, nanosecond optical pulses with high power and spectral brightness in the deep ultraviolet (UV) region have been produced by sum frequency mixing of nearly transform-limited-bandwidth IR light originating from a home-built injection-seeded ring cavity KTiOPO(4) optical parametric oscillator (OPO) and the fourth harmonic beam of an injection-seeded Nd:YAG laser used simultaneously to pump the OPO with the second harmonic. We demonstrate UV output, tunable from 204 to 207 nm, which exhibits pulse energies up to 5 mJ with a bandwidth better than 0.01 cm-1. We describe how the approach shown in this paper can be extended to wavelengths shorter than 185 nm. The injection-seeded OPO provides high conversion efficiency (> 40% overall energy conversion) and superior beam quality required for highly efficient downstream mixing where sum frequencies are generated in the UV. The frequency stability of the system is excellent, making it highly suitable for optical pumping. We demonstrate high resolution spectroscopy as well as optical pumping using laser-induced fluorescence and stimulated emission pumping, respectively, in supersonic pulsed molecular beams of nitric oxide.