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Fourier Transform Limited IR Source

Fachliche Zuordnung Physikalische Chemie
Förderung Förderung in 2011
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 191296131
 
Erstellungsjahr 2015

Zusammenfassung der Projektergebnisse

The main theme of our research is to study molecule-surface interaction at a very fundamental level, focussing on the conversion of energy between translational, vibrational and rotational motion of the molecule, and phonon and electron-hole pair excitation of the solid. In particular, collision-induced vibrational excitation and relaxation are known to be strongly influenced by nonadiabatic effects, i.e. coupling between nuclear and electronic motion. We use crystalline surfaces under ultra-high vacuum, molecular beams, and various laser-based detection schemes to extract state-to-state vibrational excitation and relaxation probabilities. In the context of such experiments, the Fourier transform limited IR source is used for vibrational overtone pumping of diatomic molecules, specifically NO, CO and HCl. With NO, the system was used to pump the X2 Π v=0→2 and v=0→3 transitions. The required wavelengths near 2.68 µm (3729 cm^-1) and 1.80 µm (5549 cm^-1) were generated as the idler and signal wave, respectively, of the optical parametric amplification stage of the laser system. With CO, it was used to pump the X1 ∑+ v=0→2 transition with wavelengths near 2.35 µm (4264 cm^-1). Currently, the system is being used with HCl molecules to pump the X1 ∑+ v=0→1 and v=0→2 transitions with wavelengths near 3.44 µm (2906 cm^-1, idler) and 1.76 µm (5688 cm^-1, signal), respectively. For all of these systems, the narrow linewidth of the Fourier transform limited IR source, measured as 130±10 MHz, is essential to obtaining high pumping efficiency. Scientifically, vibrational overtone pumping has served three different purposes: 1. Many of the phenomena under study depend on the translational energy of the incident molecule when it hits the surface. The velocities of molecules in the molecular beam can be tuned by using different carrier gases and varying their concentrations. The velocity distributions were measured using a time-of-flight technique, where vibrational overtone excitation is used for "tagging” molecules in the beam. These molecules are then state-specifically detected using a pulsed dye laser for resonant ionization (REMPI). The velocity of the molecules is determined with high accuracy from the spatial offset and temporal delay between the two laser pulses. 2. State-to-state measurements of collision-induced vibrational relaxation carry complementary information about the nonadiabatic coupling between vibrational and electronic degrees of freedom, with much higher signal-to-noise ratio than vibrational excitation measurements. For these experiments, vibrational overtone pumping is used to prepare incident NO molecules in vibrationally excited states v=2,3. After scattering from the surface, their rotational and vibrational state distributions are probed using REMPI. In addition, translational energy distributions can be measured with a time-of-flight technique, providing information on the coupling between translation, vibration and rotation. 3. Steric effects in molecule-surface collision were investigated by measuring vibrational relaxation probabilities in NO scattering from Au(111), with controlled orientation of the NO molecules at the instant of collision with the surface. Orientation was achieved with a recently developed technique, optical state selection with adiabatic orientation in a strong electric field. In these experiments, vibrational overtone pumping is used to prepare NO molecules in one or the other component of the X2 Π v=3 e/f ∆ doublet - these states evolve into oriented states as the molecule enters the electric field. The narrow linewidth of the Fourier transform limited IR source is essential here as the splitting of the two components is only ~900 MHz.

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