Experimental Methods: Generation of Cold Gas-Phase Molecules, Molecular Ions, Their Clusters, Metal Clusters, and Laser Spectroscopy

Abstract

In this chapter, we describe the methods of generating cold neutral and ionic (cation and anion) molecules, their clusters, and metal clusters in the gas-phase. First, a technique of supersonic free-jet or supersonic beam to generate cold neutral molecules and clusters is described. In addition, heating and laser ablation nozzles for the geneation of supersonic free-jet of nonvolatile molecules, such as high melting point and bio-related molecules, are introduced, while the methods of laser ablation and magnetron sputtering to generate metal clusters are also described. We then introduce various laser spectroscopic methods to measure the electronic and vibrational spectra for the jet-cooled molecules. Laser-induced fluorescence (LIF) and resonance-enhanced two-photon ionization (R2PI) spectroscopy is used to measure the electronic spectrum. UV–UV hole-burning (UV-UV HB) spectroscopy is used to discriminate the electronic transitions of different conformers and isomers. For the measurement of the vibrational spectrum of a specific molecule or cluster, we apply infrared-ultraviolet double-resonance (IR-UV DR) spectroscopy. If the molecule has no chromophore, a combination of IR and vacuum UV laser light (IR-VUV) is used to obtain the vibrational spectrum. Second, we describe the generation methods of gas-phase cold ionic molecules and clusters. The gas-phase ions are generated by resonant-enhanced multi-photon ionization, electron impact, electron attachment, matrix-assisted laser disorption/ionization (MALDI), and electrospray ionization (ESI). Cooling of the ions is achieved by supersonic expansion or by the use of cryogenically cooled ion-trap. A time-of-flight (TOF) mass spectrometry or quadrupole mass filter is used for the mass selection, which is also applicable to obtain the single-sized metal clusters selectively. To obtain the electronic and vibrational spectra of the ionic species, we apply UV photodissociation (UVPD) and IR multi-photon dissociation (IRMPD), respectively. IR-UV DR spectroscopy is also used to measure the IR spectrum of a specific ion. In addition to the detection of the ions, a measurement of the photo-ejected electron, called photoelectron spectroscopy, is also described. Finally, we introduce pump–probe spectroscopy to investigate the dynamics of the vibrationally and electronically excited molecules and clusters.