Instrumentation
Research in the de Vries Group specializes in cold gas phase spectroscopy by utilizing laser desorbed, jet cooled, mass spectrometry utilizing Resonance Enhanced Mutli-Photon Ionization(REMPI). This powerful technique combines laser spectroscopy and mass spectrometry to offer unparalleled specificity when ionizing a particular compound and vibronic level spectroscopic information.
This technique can be broken down into 4 steps: (1) laser desorption, (2) supersonic jet cooling, (3) Resonance Enhanced Multi-Photon Ionization, and (4) time-of-flight mass spectrometry.
Solid samples are placed on a graphite bar. The fundamental of an Nd:YAG laser (1064nm) is absorbed by the graphite bar and rapidly heated causing the solid sample to thermally desorb. The desorption event as pictured in the above figure takes place directly in front of a pulsed valve that generates a molecular beam of argon. This molecular beam is responsible for collisionally cooling the sample down to a vibrational temperature of 10K. Molecules in the cold gas phase will have only the lowest ground state vibrational bands populated, greatly narrowing the allowed excited state transitions. For this reason, instead of broad absorption bands being observes as those seen in solution phase spectroscopy, individual vibronic bands can be easily resolved.
The cold, desorbed sample will then be ionized through REMPI. REMPI ionizes in a two photon process where the first photon is resonant with the excited state of the analyte and the second photon ionizes out of the excited state. This can be done as either 1- or 2- color with 2-color most often being used. By utilizing two separate laser for the two photons, a lot of possibilities are available. Pump probe technique can be used to establish excited state lifetimes by varying the time between the excitation(pump) and the ionization (probe) laser. As the time delay increases between the two photons, the excited state is given time to decay producing a decay in signal. This decay in signal can be fitted to produce a lifetime for that particular excited state. This can be done for each separate vibronic band in the spectra. Adding an additional laser allows for double resonance techniques which can be used to determine the number of tautomers present in a spectra and also determine the IR spectra of a particular tautomer. After a sample is ionized it is detected by a TOF- mass spectrometer.
Laser Systems
Our combination of lasers allows for elucidation of photo-dyamics in both the picosecond and nanosecond regimes. With these multiple laser systems, we are capable of doing REMPI (1- or 2- color with 213nm, 266nm or 193nm available as the second color), double resonance techniques(UV-UV, IR-UV), and pump probe.
