Research

Photochemistry

 

Rydberg H-Atom Photofragment Translational Spectroscopy   
    The Rydberg H-atom translational spectroscopy technique was developed in the early 1990s by Welge and coworkers.(J.Chem.Phys.1990,92,7027).This technique has provided us with an extremely powerful tool for measuring the state-resolved differential cross-sections for both unimolecular and bimolecular reactions with unprecedented translational energy resolution and extremely high sensitivity.This technique has been applied successfully to studies of the important benchmark reactions H+D2 HD+H, O(1D)+H2 OH+H recently and many important unimolecular dissociation processes in our laboratory. These state-of the-art experimental studies, coupled with the recent advances in theoretical state-to-state dynamics studies,can now provide an in-depth physical understanding of elementary chemical reactions that could not be imagined before.

The experimental set-up is shown below

 

Advantages of Rydberg Tagging    
    Many techniques used for studying photodissociation processes rely on the detection of ions which are produced either directly, or by ionisation of neutral photofragments in the volume where photodissociation occurs (the interaction region). The formation of ions leads to a high detection efficiency, as the use of correctly tuned electric fields (ion optics) may be used to accelerate them toward a detector. However, the formation of many charged species in the small focal volume of the interaction region of the experiment (in this case the focus of a laser beam) can lead to blurring of the resulting data due to Coulomb repulsion forces. In order to circumvent this problem, the photofragments can be initially excited into certain high energy electronic (Rydberg) neutral states that have long (ms) lifetimes and are just below (in energy) an Ionisation Potential. These "Rydberg tagged", but still neutral photofragments can then move with unperturbed velocities to the ion detector where a small electric field is present and (only) ionises the tagged photofragments immediately prior to detection.

    Recently, we developed a new method to generate two VUV beams in one four -wave mixing cell. One is fixed at 121.6nm and the other is tunable from 120-190nm.Now we can investigate tunable ultraviolet photo-excitation as well as vacuum ultraviolet photo-excitation of molecules with high resolution. For example, vacuum ultraviolet photo-excitation of methane leads to fragmentation. What products are formed, and with what quantum yields? We were able to provide the first definitive answers to these questions by measuring the times-of-flight, and thus the kinetic energies, of the resulting H atom photofragments. The method relies on energy conservation: H atoms formed in association with internally excited partner fragments (e.g. vibrationally excited CH3 radicals) must possess less kinetic energy. The H atom kinetic energy spectrum thus carries an imprint of the population distribution of the (unobserved) partner fragment - precisely the information sought! Other molecules whose primary photochemistry we have studied in this way include the hydrogen halides, HCN, H2O, H2S, NH3, PH3, C2H2, CH3SH, CH3NH2, HCOOH, HCHO, HFCO, HN3, allene (CH2CCH2), propyne (CH3CCH), ketene (H2CCO) and the CH3 radical. Current activities are centred on the photolysis of H2O In the different excited electronic states.
The figure below shows the H-atom PTS spectra recorded upon H2O photodissociation at 121.6nm. (science,1999,285,1249)

Figure. 3D Product (H) Contour Diagram from H2O Photodissociation at 121.6 nm (0 – 25,000 cm-1)

 
 

 

Copyright 2008-2009, Group 1102, DICP. All Rights Reserved.