Spectroscopy optothermal

For the spectroscopy of vibrational-rotational transitions in molecules the laser-excited fluorescence is generally not the most sensitive tool, as was discussed at the end of Sect. 1.3.1. Optoacoustic spectroscopy, on the other hand, is based on colli-sional energy transfer and is therefore not applicable to molecular beams, where collisions are rare or even completely absent. For the infrared spectroscopy of molecules in a molecular beam therefore a new detection technique has been developed, which relies on the collision-free conditions in a beam and on the long radiative lifetimes of vibrational-rotational levels in the electronic ground state [84-86]. 

Optothermal spectroscopy uses a cooled bolometer (Vol. 1, Sect. 4.5) to detect the excitation of molecules in a beam (Fig. 1.30). When the molecules hit the bolometer they transfer their kinetic and their internal thermal energy, thereby increasing the bolometer temperature Tq by an amount AT. If the molecules are excited by a tunable laser (for example, a color-center laser or a diode laser) their vibrational-rotational energy increases by A = /iv kin- If the lifetime r of the excited levels is larger than the flight time t = d v from the excitation region to the bolometer, they transfer this extra energy to the bolometer. If N excited molecules hit the bolometer per second, the additional rate of heat transfer is 

1 Doppler-Limited Absorption and Ruorescence Spectroscopy with Lasers 

With the heat capacity C of the bolometer and a heat conduction G T — To) the temperature T is determined by 

Under stationary conditions (dr/dr = 0), we obtain from (1.42) the temperature rise 

Doppler-Limited Absorption and Fluorescence Spectroscopy with Lasers 

In general, the exciting laser beam is chopped in order to increase the signal/noise ratio by lock-in detection. The time constant r = C/G of the bolometer (Sect.4.5) should be smaller than the chopping period. One therefore has to construct the bolometer in such a way that both C and G are as small as possible. 

The temperature change AT can be measured by the resulting resistance change 

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Fraser, G. T. and Pine, A. S., Microwave and infrared electric-re.sonance optothermal spectroscopy ofHF-HCl and HCl-HF, J. Chem. Phys. 91,637-645 (1989). 

The term action spectroscopy refers to those techniques that do not directly measure the absorption, but rather the consequence of photoabsorption. That is, there is some measurable change associated with the absorption process. There are several well known examples, such as photoionization spectroscopy [42], multiphoton ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [50, 51], vibrational predissociation spectroscopy [52] and optothermal spectroscopy [53, M]- These techniques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

Optothermal spectroscopy is a bolometric method that monitors the energy in a stream of molecules rather than in the light beam. A well collimated molecular beam is directed toward a liquid helium cooled bolometer. There will be energy... 

Fig. 1.30 Optothermal spectroscopy in a molecular beam with a helium-cooled bolometer as detector and two optical systems, which increase the absorption path length...

The sensitivity of the optothermal technique is illustrated by the comparison of the same sections of the overtone spectrum of C2H4 molecules, measured with Fourier, optoacoustic, and optothermal spectroscopy, respectively (Fig. 1.34). Note the increase of spectral resolution and signal-to-noise ratio in the optothermal spectrum compared to the two other Doppler-limited techniques [87]. More examples can be found in [88]. 

Fig. 1.35 Optothermal spectroscopy of solids and of molecules adsorbed at surfaces (a) propagation of thermal wave induced by a pulsed laser (b) deformation of a surface detected by the deflection of a HeNe laser beam (c) time profile of surface temperature change following pulsed illumination [90]...

For infrared spectroscopy in molecular beams optothermal spectroscopy is a very good choice (Sect. 1.3.3). 

In particular, the various applications of laser spectroscopy in physics, chemistry, biology, and medicine, and its contributions to the solutions of technical and environmental problems are remarkable. Therefore, a new edition of the book seemed necessary to account for at least part of these novel developments. Although it adheres to the concept of the first edition, several new spectroscopic techniques such as optothermal spectroscopy or velocity-modulation spectroscopy are added. 

The main part of the book presents various applications of lasers in spectroscopy and discusses the different methods that have been developed recently. Chapter 6 starts with Doppler-limited laser absorption spectroscopy with its various high-sensitivity detection techniques such as frequency modulation and intracavity spectroscopy, cavity ring-down techniques, excitation-fluorescence detection, ionization and optogalvanic spectroscopy, optoacoustic and optothermal spectroscopy, or laser-induced fluorescence. A comparison between the different techniques helps to critically judge their merits and limitations. 

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