Optical double resonance sensitivity

K. Yamanouchi Recently, we investigated the interatomic potential VRyd(/ ) of the Rydberg states of a HgNe van der Waals dimer by optical-optical double-resonance spectroscopy. It was demonstrated that VRyd(/ ) sensitively varies as a function of the principal quantum number n [J. Chem. Phys., 98, 2675 (1993) ibid., 101, 7290 (1995) ibid., 102, 1129 (1995)], and in the lowest Rydberg states of Hg(7 3S )Ne and Hg(7 5o)Ne, the interatomic potentials exhibit a distinct barrier at around R 4 A. The existence of the barrier was interpreted in terms of a repulsive interaction caused by the Is Rydberg... 

Millimeter wave spectroscopy with a free space cell such as a Broida oven is more sensitive than lower frequency microwave spectroscopy. However, the higher J transitions monitored by millimeter wave spectroscopy often do not show the effects of hyperfine structure. In the case of CaOH and SrOH, the proton hyperfine structure was measured in beautiful pump-probe microwave optical double resonance experiments in the Steimle group [24,68], They adapted the classic atomic beam magnetic resonance experiments to work with a pulsed laser vaporization source and replaced the microwave fields in the A and C regions by optical fields (Fig. 15). These sensitive, high-precision measurements yielded a very small value for the proton Fermi contact parameter (bF), consistent with ionic bonding and a... 

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-... 

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]. 

Sensitivity of optical double-resonance experiments. In conventional solid state magnetic resonance experiments the necessary population difference between the states is created by the Boltzmann factor exp(jj.B/kT) and is enhanced by working at low temperatures and high field strengths. 

By contrast optical double-resonance experiments can be performed with vapour densities as low as 10° atoms cm". This great increase in sensitivity is due to the high atomic polarization achieved by optical excitation combined with the fact that in these experiments the absorption of an r.f. quantum triggers the detection of a visible or ultraviolet quantum whose energy is some 10° - 10 times greater. Optical double-resonance experiments can therefore be performed on samples containing only a few milligrams of mass-... 

EPR but provides greatly enhanced resolution. Double resonance techniques (e.g. electron nuclear double resonance (ENDOR) and Overhauser shift measurements) combine the sensitivity of EPR with the resolution of NMR. Many such measurements on thin films are performed by combining optical detection with ENDOR, greatly enhancing the resolution of ODMR and taking advantage of its superior sensitivity. 

For a given value of B, the energies of Am/ = 1 transitions between the nuclear sublevels of a given electronic spin state are much lower than those between the electronic spin components. Information on the amplitude of the wave function of the electron whose spin is responsible for the ESR spectrum at different lattice sites in the vicinity of the centre was obtained by Feher [17] by monitoring the ESR spectrum as a function of the frequencies in the nuclear frequency range, and this technique was called electron nuclear double resonance (ENDOR). Improvements in the sensitivity of ESR can be obtained using optical or electrical detection methods [47]. 

Slow-passage ODMR signals frequently are observed by the continuous wave method in which the optical effect is monitored using broadband detection. On the other hand, if the triplet state decay constants are sufficiently large, the microwave power may be amplitude modulated at an audio frequency which results in modulated phosphorescence when the microwave frequency is at resonance. The phosphorescence is then monitored with narrow-band phase-sensitive detection, for a great improvement in the signal/noise ratio. The latter detection method is frequently used to produce a magnetic resonance-induced phosphorescence spectrum by a technique referred to as phosphorescence-microwave double resonance (PMDR). The microwave frequency is fixed at resonance,... 

Each absorbed RF photon leads to an extra absorbed optical photon of the pump beam. This optical-RF double resonance therefore yields an internal energy amplification factor V = ct>opt/< rf for the detection of an RF transition. With = 3 X 10 Hz and co f =10 Hz, we obtain V = 3 x 10 Since optical photons can be detected with a much higher efficiency than RF quanta, this inherent energy amplification results in a corresponding increase in the detection sensitivity. 

Fig. 5.11 Atomic beam resonance apparatus with combined electron-impact and laser pumping for the sensitive detection of optical-RF double resonance in highly excited states [529]...

An interesting application of optical-RF double resonance is the realization of sensitive magnetometers. Here a cell with rubidium vapor at room temperature is placed in a magnetic fleld and the RF is tuned to transitions between Zeeman components in the state. Since the Lande factors are known, the magnetic field strength can be obtained by measuring the radiofrequency [530]. 

The electronically excited states of most molecules are far less thoroughly investigated than their ground states. On the other hand, their level structure is generally more complex because of interactions between electron and nuclear motions, which are more pronounced in excited states (breakdown of the Born-Oppenheimer approximation, perturbations). It is therefore most desirable to apply spectroscopic techniques that are sensitive and selective and that facilitate assignment. This is just what the optical-microwave double-resonance technique can provide. 

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. 

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