Modulated Population Spectroscopy

Questions of linkage are posed and answered by asking the molecule to satisfy successively two resonance conditions. Schemes which accomplish this include Dispersed Fluorescence Spectroscopy (DF, Section 1.2.2.2 a laser is tuned to excite a single line and the spectrum of the resulting molecular fluorescence is recorded), Modulated Population Spectroscopy (MPS, Section 1.2.2.3) an intense, fixed frequency, amplitude modulated PUMP laser is used to modulate the population in the upper and lower levels connected by the laser excited transition the modulation is then detected by a frequency scanned PROBE laser), which is an example of Optical Optical Double Resonance (OODR, Section 1.2.2.3). 

Laser excitation here offers few distinctive features at the molecular level, except that at high levels of intensity the increased flux can lead to saturation. Then, a significantly high proportion of the sample molecules undergoes transition to an excited state and, as C varies through depletion of the ground state population, departures from Beer s law arise. This is a phenomenon that is exploited for analytical purposes with pulsed radiation, in concentration-modulated absorption spectroscopy. 

Photoacoustic spectroscopy is based on measurement of the heat generated by the radiationless processes for the deactivation of excited molecules (Rosencwaig, 1980). The population of the excited species and the heat emission will be modulated with the same frequency as the exciting light source. At the appropriate frequencies, the excited species will emit their excess heat phase-shifted with respect to the heat emitted by the fast relaxation processes. A gas-coupled microphone and a phase-sensitive detector in combination with an intensity-modulated light source can be... 

In the excited state n the interaction of the radiation field with the molecule induces emission, and the molecule relaxes to the lower state m. The induced emission is indistinguishable from the field that caused it. Furthermore, it is of no use analytically unless the excitation source radiation is interrupted before the upper state population has had time to relax. That becomes the case in Pulsed Fourier Transform microwave spectroscopy where the excitation source is pulse modulated and the induced radiation is emitted against a very low microwave background. 

Since the foundations of atomic absorption spectroscopy were laid by Walsh a number of improvements in instrumentation and techniques have been made. Russell, Shelton, and Walsh modulated the hollow cathode signal and used an amplifier tuned to the modulating frequency so measurements could be made without interference from flame emission. Sullivan and Walsh developed very high-intensity hollow cathode lamps that led to lower detection limits. Willis proposed the use of nitrous oxide-acetylene flame as a means of overcoming certain interferences and produce a higher population of free atoms in the flame. 

We have already discussed quantum-beat spectroscopy (QBS) in connection with beam-foil excitation (Fig.6.6). There the case of abrupt excitation upon passage through a foil was discussed. Here we will consider the much more well-defined case of a pulsed optical excitation. If two close-lying levels are populated simultaneously by a short laser pulse, the time-resolved fluorescence intensity will decay exponentially with a superimposed modulation, as illustrated in Fig. 6.6. The modulation, or the quantum beat phenomenon, is due to interference between the transition amplitudes from these coherently excited states. Consider the simultaneous excitation, by a laser pulse, of two eigenstates, 1 and 2, from a common initial state i. In order to achieve coherent excitation of both states by a pulse of duration At, the Fourier-limited spectral bandwidth Au 1/At must be larger than the frequency separation ( - 2)/ = the pulsed excitation occurs at... 

This optical-optical double-resonance technique has already been used for other Doppler-free techniques [10.25], such as polarization spectroscopy (see Sect.10.3). Its applications to molecular beams has, however, the following advantages compared to spectroscopy in gas cells. When the chopped pump laser periodically depletes the level E. and populates level Ej, there are two relaxation mechanisms in gas cells which may transfer the population modulation to other levels. These are collision processes and laser-induced fluorescence (see Fig.8.39). The neighboring levels therefore also show a modulation and the modulated excitation spectrum induced by the probe laser includes all lines which are excited from those levels. If several absorption lines overlap within their Doppler width, the pump laser simultaneously excites several upper states and also partly depletes several lower levels. 

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