Supersonic cooling spectroscopy

The intensely developing technique of high-resolution IR-spectroscopy of dimers composed of two different molecules in supersonic cooled jets offers a new promising approach to the quantum dynamics of reaction complexes. In essence, this is a unique possibility of modelling low-temperature chemical reactions. 

Cold, with well-defined internal energy. This is normally ensured with supersonic cooling. This requirement is important for high resolution and state-resolved studies, and it can remove the spectroscopy congestion of the radicals and enhance the interpretation of the results in photochemistry experiments. 

A universal method that satisfies both requirements is supersonic cooling. For this reason, this method is a cornerstone of modern low-temperature studies. The results described in this chapter are mainly obtained with this method. For special reviews devoted to applications of supersonic cooling in molecular spectroscopy we refer the reader to the papers by Levy [1980] and Cohen and Saykally [1992]. Technical details of the method can be found in the papers we cite in this chapter. 

The experimental apparatus, as shown in Figure 11-1, was a standard molecular beam machine with a heated pulsed valve for vaporization of the non-volatile species and for supersonic cooling. Samples of 1-methyluracil, 1,3-dimethyluracil and thymine were purchased from Aldrich Co. and used without further purification. The sample 1,3-dimethylthymine was synthesized from thymine following a literature procedure [33], and its purity was checked by nuclear magnetic resonance (NMR) and infrared absorption (IR) spectroscopy. The heating temperatures varied for different samples 130°C for DMU, 150°C for MU, 180°C for DMT, and 220°C for thymine. No indication of thermal decomposition was observed at these... 

Fig. 4. An ortho-methyl group in diethylamino-pyrimidin induces some ground state twist and hence energetically destabilizes the B state but not yet sufficiently to make the population of the A state a najor process in supersonic jet spectroscopy. Upper panel dispersed fluorescence spectra of the jet-cooled bare molecule [36]. In clusters with methanol, the TICT state is preferentially lowered, and the majority of the ob rved red-shifted fluorescence can be assign l to arise from the TICT state (lower panel). This does not occur for the compound without an ortho-methyl group.

We use laser photofragment spectroscopy to study the vibrational and electronic spectroscopy of ions. Our photofragment spectrometer is shown schematically in Eig. 2. Ions are formed by laser ablation of a metal rod, followed by ion molecule reactions, cool in a supersonic expansion and are accelerated into a dual TOE mass spectrometer. When they reach the reflectron, the mass-selected ions of interest are irradiated using one or more lasers operating in the infrared (IR), visible, or UV. Ions that absorb light can photodissociate, producing fragment ions that are mass analyzed and detected. Each of these steps will be discussed in more detail below, with particular emphasis on the ions of interest. 

This chapter deals mainly with (multi)hyphenated techniques comprising wet sample preparation steps (e.g. SFE, SPE) and/or separation techniques (GC, SFC, HPLC, SEC, TLC, CE). Other hyphenated techniques involve thermal-spectroscopic and gas or heat extraction methods (TG, TD, HS, Py, LD, etc.). Also, spectroscopic couplings (e.g. LIBS-LIF) are of interest. Hyphenation of UV spectroscopy and mass spectrometry forms the family of laser mass-spectrometric (LAMS) methods, such as REMPI-ToFMS and MALDI-ToFMS. In REMPI-ToFMS the connecting element between UV spectroscopy and mass spectrometry is laser-induced REMPI ionisation. An intermediate state of the molecule of interest is selectively excited by absorption of a laser photon (the wavelength of a tuneable laser is set in resonance with the transition). The excited molecules are subsequently ionised by absorption of an additional laser photon. Therefore the ionisation selectivity is introduced by the resonance absorption of the first photon, i.e. by UV spectroscopy. However, conventional UV spectra of polyatomic molecules exhibit relatively broad and continuous spectral features, allowing only a medium selectivity. Supersonic jet cooling of the sample molecules (to 5-50 K) reduces the line width of their... 

A technique which is not a laser method but which is most useful when combined with laser spectroscopy (LA/LIF) is that of supersonic molecular beams (27). If a molecule can be coaxed into the gas phase, it can be expanded through a supersonic nozzle at fairly high flux into a supersonic beam. The apparatus for this is fairly simple, in molecular beam terms. The result of the supersonic expansion is to cool the molecules rotationally to a few degrees Kelvin and vibrationally to a few tens of degrees, eliminating almost all thermal population of vibrational and rotational states and enormously simplifying the LA/LIF spectra that are observed. It is then possible, even for complex molecules, to make reliable vibronic assignments and infer structural parameters of the unperturbed molecule therefrom. Molecules as complex as metal phthalocyanines have been examined by this technique. 

Although this book is devoted to molecular fluorescence in condensed phases, it is worth mentioning the relevance of fluorescence spectroscopy in supersonic jets (Ito et al., 1988). A gas expanded through an orifice from a high-pressure region into a vacuum is cooled by the well-known Joule-Thomson effect. During expansion, collisions between the gas molecules lead to a dramatic decrease in their translational velocities. Translational temperatures of 1 K or less can be attained in this way. The supersonic jet technique is an alternative low-temperature approach to the solid-phase methods described in Section 3.5.2 all of them have a common aim of improving the spectral resolution. 

A problem with high resolution spectroscopy is the analysis of the enormous amounts of very complicated data FT spectrometers can produce. If the spectrum is taken at or above room temperature many rotational states of the radical will be populated and a huge number of transitions will be seen (Figure 7). One means of simplifying the spectrum is to cool the species that is being studied. A very useful way of accomplishing this is to seed the target radical in a supersonic expansion of... 

Two-colour photoionization spectroscopy of aniline cooled in a supersonic jet. Strong propensity for vertical (An = 0) ionization allows vibrational frequencies of CgHgNH2 ( B,) to be determined Two-colour photoacoustic and MPI spectra of aniline, determined as a function of time delay between the two laser pulses. Observed both ionization and dissociation t MPI/TOF mass spectrometric study of phenol. Mechanism of ionization and ion fragmentation t MPI/TOF mass spectrometric study of fragmentation patterns in benzaldehyde. Strong wavelength dependence observed at 266 and 355 nm. Results show operation of two different mechanisms at these excitation wavelengths... 

Most investigations of photoinduced electron transfer have been performed in condensed phases. Much less is known about conditions that permit the occurrence of intramolecular ET in isolated (collision-free) molecular D-A systems. A powerful method for this kind of study is the supersonic jet expansion teehnique (which was originally developed by Kantrowitz and Grey in 1951 [66]) combined with laser-induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF-MS). On the other hand, the molecular aspects of solvation can be studied by investigations of isolated gas-phase solute-solvent clusters which are formed in a supersonic jet expansion [67] (jet cooling under controlled expansion conditions [68] permits a stepwise growth of size-selected solvation clusters [69-71]). The formation of van der Waals complexes between polyatomic molecules in a supersonic jet pro-... 

A few years ago, a powerful version of molecular optical spectroscopy with supersonic beams and jets was developed by Smalley, Wharton and Levy . Supersonic expansion of molecules in an inert carrier gas yields an ideal spectroscopic sample. As a result of the expansion, the translational temperature of the carrier gas decreases to extremely low values (below O.I K). The flow is collisionless so that even extremely unstable species survive. Special attention was paid to fluorescence excitation spectroscopy but the technique is by no means limited to this type of spectroscopy. (Because of fundamental difficulties, however, direct measurement of light absorption in molecular beams is not easy.) Cooled molecules in the beam are electronically excited with a tunable dye laser. The emitted fluorescence is detected and plotted against the wavenumber of the exciting radiation. The obtained fluorescence excitation spectrum is generally very similar to the corresponding absorption spectrum. The technique was used for analysis of the spectra of interesting vdW molecules He. .. NOj, He... Ij, X. .. tetrazine and Xj. .. tetrazine (X = He, Ar, H ) complexes . 

Other methods for dealing with a complex spectrum include simplifying it by cooling the molecule to 5 K in a supersonic jet (Section 1.2.3) or selectively enhancing the relative intensities of certain low-7 rotational lines (Magnetic Rotation Spectroscopy, Section 1.2.5). 

A supersonic jet obtained by the expansion of a gas through a nozzle differs from the traditional bulb sample in two important respects. First, the gas flow is directed in space with a velocity distribution that can be quite narrow. Second, the sample translational temperature is cooled to a few degrees Kelvin and many of the internal degrees of freedom are also cooled substantially. This is especially important for large molecules which have many vibrational modes excited at room temperature. The spectroscopy of even large molecules is thus simplified to the point where it is possible to excite single vibrational states and in some cases rotational levels as well. 

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