Polarization spectroscopy sensitivity

Steady-state and multifrequency phase and modulation fluorescence spectroscopy are used to study the photophysics of a polar, environmentally-sensitive fluorescent probe in near- and supercritical CF3H. The results show strong evidence for local density augmentation and for a distribution of cluster sizes. These results represent the first evidence for lifetime distributions in a "pure solvent system. 

An extremely sensitive MODR scheme, microwave optical polarization spectroscopy (MOPS), was introduced by Ernst and Torring (1982). The most important features of MOPS are that it requires respectively 100 and 10 times lower laser and microwave intensities than MODR and results in 10 times narrower lines. This means that it will be possible to take full advantage of differential power broadening effects (Section 6.5.1) and to utilize low-power, frequency-doubled dye lasers and low-power, broadly tunable microwave sources (backward wave oscillators) in order to gain access to and systematically study perturbations. 

Another coherent technique sensitive to combustion conditions is polarization spectroscopy (PS) [7,9]. Ffere, a weak linearly polarized probe laser beam is crossed... 

The sensitivity of polarization spectroscopy compared with saturation spectroscopy is illustrated by Fig. 2.24a, which shows the same hfs transitions of I2... 

In the following we briefly discuss the sensitivity and the signal-to-noise ratio achievable with polarization spectroscopy. The amplitude of the dispersion signal in (2.50) for 0 is approximately the difference AIj = It x = +1) — Ij x = — 1) between the maximum and the minimum of the dispersion curve. From (2.50) we obtain (Fig. 2.29)... 

The higher sensitivity of polarization spectroscopy compared with conventional saturation spectroscopy results from the detection of phase differences rather than amplitude differences. This advantage is also used in a method that monitors the interference between two probe beams where one of the beams suffers saturation-induced phase shifts. This saturated interference spectroscopy was independently developed in different laboratories [271, 272]. The basic principle can easily be understood from Fig. 2.43. We follow here the presentation in [271]. 

The sensitivity of the saturated interference technique is comparable to that of polarization spectroscopy. While the latter can be applied only to transitions from levels with a rotational quantum number / > 1, the former works also for 7=0. An experimental drawback may be the critical alignment of the Jamin interferometer and its stability during the measurements. 

A very sensitive and accurate double-resonance technique is microwave-optical double-resonance polarization spectroscopy (MOPS), developed by Ernst et al. 

Mode 2 is a particularly sensitive method to detect mw transitions as will be shown in chapter III and is called microwave-optical polarization spectroscopy (MOPS). Polarization spectroscopy techniques require less intensity of laser and mw radiation than the corresponding nonlinear methods based on fluorescence detection. Power broadening is avoided which is the reason for the largely improved resolution of MOPS compared to conventional microwave optical double resonance (MOOR) spec-... 

The sensitivity of polarization spectroscopy compared with saturation spectroscopy is illustrated by Fig. 7.22a, which shows the same hfs transitions of I2 as in Fig. 7.12 taken under comparable experimental conditions. A section of the same spectrum is depicted in Fig. 7.22b with 9 7 0, optimized for dispersion-line profiles. 

Major advantages of microwave-optical polarization spectroscopy are narrower linewidths as compared to conventional laser-rf double resonance and smaller intensities required for the laser light field and the micro-waves, so that strongly saturating conditions can be avoided. Therefore, the sensitivity as well as the resolution can be greatly enhanced. 

A new very sensitive and accurate double-resonance technique is the Microwave-Optical double-resonance Polarization Spectroscopy (MOPS) developed by Ernst et. al [10.93]. This technique detects microwave transitions in a sample between crossed polarizers through the change in transmission of a polarized optical wave. The sensitivity of the method has been demonstrated by measurements of the hfs of rotational transitions in the electronic ground state of CaCl molecules which were produced by the reaction 2Ca+ CI2 - CaCl in an argon flow. In spite of the small concentrations of CaCl reaction products and the short absorption pathlength in the reaction zone a good signal-to-noise ratio could be achieved at linewidths of lf2 MHz [10.94]. 

Various techniques of sensitive absorption spectroscopy, including nonlinear techniques, which allow a spectral resolution below the Doppler width are described first. These techniques are termed sub-Doppler-spectroscopy and include linear spectroscopy in collimated molecular beams, nonlinear saturation and polarization spectroscopy, and Doppler-free two-photon spectroscopy. Emission spectroscopy, which covers laser-induced fluorescence as well as stimulated emission methods, is described next. The assignment of complex molecular... 

Orientation changing collisions can be sensitively detected with polarization spectroscopy (see Sect,10.3). Since the polarized pump wave produces a partial orientation of the molecular angular momentum, which gives rise to a change in polarization of the linearly polarized probe wave, any collisions which change the orientation alter the probe intensity transmitted by the second polarizer in Fig.10.33. Another way to detect these collisions uses saturation spectroscopy, where the angle between the planes of polarization of pump and probe wave are varied and the Lamb dip profile is monitored as a function of 3 [12.6]. 

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