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Under the cold and isolated conditions of the supersonic expansion, spectroscopic transitions are highly resolved due to the absence of common broadening mechanisms. With that in mind, the light source used must be highly monochromatic (sharp bandwidth), yet wavelength tunable. In our lab, we use pulsed Nd:YAG lasers as pump sources for other tunable lasers/oscillators to render those conditions. When the laser pulse intersects the cold molecular beam, the molecule becomes excited only when the wavelength of the laser matches exactly the energy difference between ground and excited states. Once the molecule is excited, we have two common ways of detecting it.
Resonant Two-Photon Ionization (R2PI)
With the high intensity laser pulses used in these experiments, there is a large surplus of photons within each laser pulse used to excite the sample. After the laser-induced excitation, the molecule is transiently storing a significant amount of energy thereby bringing the molecule closer to its ionization potential. The extra photons in the pulse provide the excess energy to eject an electron from the molecule to form the molecular cation. Given the ion has a well-defined positive charge, it can then be injected into a mass spectrometer where the individual ions are detected on a microchannel plate. The result is a two-dimensional spectrum consisting of a mass spectrum at each wavelength. This technique is optimal for mass-selective spectroscopy.
Laser-Induced Fluorescence (LIF)
After a particle is excited by the laser, there is always a chance that it may return to its ground state by emission of a photon. This is referred to as laser-induced fluorescence (LIF). LIF is a common alternative to R2PI due to its simplicity in detection (photomultiplier tube) and insensitivity to ionization potential. While one loses the mass-selectivity of R2PI, LIF circumvents the energy requirement to form ions.
Double-Resonance Methods
Double-Resonance Methods
UV-UV Holeburning
When there are multiple unique species present within the expansion, the R2PI/LIF spectrum will be a composite of all of them. However, since the spectral lines are so sharp in those spectra there is minimal overlap of signals from those various species. This even includes conformational isomers that may only differ by a single hydrogen bond. Holeburning methods are a way to separate all of those species/isomers within a spectrum, yielding species or isomer-specific spectra. UV-UV holeburning specifically utilizes a second UV laser with a fixed wavelength set to a transition in the R2PI spectrum that arrives before the scanning probe laser. So it serves to burn a "hole" in that ground state. Since the holeburn laser is fixed to a unique ground state (i.e. one conformer), the probe will only measure less signal when it scans through transitions from that same ground state. In this way, the spectra of the individual components of a mixture can be extracted.
Resonant Ion-Dip Infrared Spectroscopy (RIDIRS) and IR-UV Holeburning
Similar to UV-UV holeburning, an infrared laser may be used to burn a hole in the ground state of a specific species within the expansion. When the probe laser is fixed to a transition unique to one of those species, the IR laser can be scanned and a dip in the UV probe signal will occur whenever the IR pulse is resonant with a unique IR transition of that species. Hence, species and conformer-specific IR spectra can be obtained by this resonant ion-dip infrared spectroscopy (RIDIRS). When the IR wavelength is fixed to a particular transition and the probe is scanned, the IR-UV holeburning spectrum is obtained yielding a UV spectrum of an IR-selected species.
R2PI (black) and UV-UV holeburning (red and blue) of sinapic acid conformers
RIDIR spectra of sinapic acid conformers
J.C. Dean, R. Kusaka, P.S. Walsh, F. Allais, and T.S. Zwier, “Plant Sunscreens in the UV-B: Ultraviolet Spectroscopy of Jet-Cooled Sinapoyl Malate, Sinapic Acid, and Sinapate Ester Derivatives,” J. Am. Chem. Soc., 136, 14780–14795 (2014).
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