Optical zero-field transition

The technique of microwave-recovery provides crucial information about the substates involved in the ODMR transitions. For this experiment, Pd(2-thpy)2 is optically excited by a c. w. source. This leads to specific populations of the three triplet substates. At low temperature, they are thermally decoupled and thus emit according to their specific populations and their individual decay constants (e. g. see Sect. 3.1.3 and Table 2). In the microwave recovery experiment, the steady state conditions are perturbed by a microwave pulse being in resonance with the zero-field transition at 2886 MHz. Due to the microwave pulse, the populations of the two states involved are changed. Subsequently, one monitors the recovery of the emission intensity in time until the steady state situation is reached again. The microwave pulses have, for example, a duration of 20 ps and are applied repeatedly to enable a detection with signal averaging [61]. 

MHz of [Rh(bpy)3](0104)3, in the phosphorescent triplet state, upon switching on the microwave power. The oscillations occur as the microwave pulse duration is increased. Photoexcitation is near 320 nm, detection is at 456 nm temperature is 1.4 K. b Optically detected echo amplitude decay for the 2320 MHz zero-field transition of [Rh(bpy)3] (0104)3 as obtained by applying a n/2-T-n-T-nl2 pulse sequence when increasing 2r... 

From magnetic resonance spectroscopy [49] it is well-known that IB effects are adequately circumvented by the tricks of a spin echo experiment. For instance, in a two-pulse echo experiment, IB effects are averaged out and one probes spin dephasing determined by time-dependent fluctuations characteristic of HB only (and not IB). More specifically, a nll-r-n microwave pulse sequence is applied, where the first nil pulse creates a coherent superposition state for which a la = 1 and the n pulse, applied at time r after the first pulse, generates a spin coherence (the echo) at time 2r after the initial pulse. The echo amplitude is traced with r. The echo amplitude decay time is characteristic of the pure dephasing dynamics. For phosphorescent triplet states it is possible to make the echo optically detectable by means of a final nil probe pulse applied at time f after the second pulse [44]. In Fig. 3b, the optically detected echo amplitude decay for the zero-field transition at 2320 MHz of... 

As shown in Fig. 3 a, spin coherence is manifested in the optically detected transient nutation signal for [Rh(bpy)3] (0)04)3 the phosphorescent triplet state. In this experiment, one observes that the phosphorescence intensity becomes modulated as the pulse length of microwave pulses, resonant with the D - transition, is gradually increased. The modulation is evidence that the micro-wave excitation induces a spin coherence in the ensemble of molecules in the photoexcited triplet state [44]. Moreover, from the transient nutation experiment one obtains the information about the duration of the pulses needed in a spin echo experiment. In the case of the example, the n/2 pulse is 100 ns and the 71 pulse has a length of 200 ns. Similarly, transient nutation signals for the other zero-field spin resonances could be obtained. The optically detected spin echo decay as measured for the D - j j zero-field transition for [Rhlbpylj](004)3... 

A second direct optical-detection method for selective population and depopulation is microwave-induced delayed phosphorescence in zero field (Bq = 0) [25]. Figure 7.26 shows the phosphorescence intensity from quinoline in a durene (tet-ramethyl benzene) host crystal at T= 1.35 K as a function of the time after the end of the UV excitation. The phosphorescing zero-field component here is Tz). Its lifetime is considerably shorter than those of the other two zero-field components, from which furthermore no phosphorescence is emitted. If the zero-field transition... 

Most of the high precision spectroscopy of He Rydberg states has been done by microwave resonance, which is probably the best way of obtaining the zero field energies. Wing et a/.8-12 used a 30-1000pA/cm2 electron beam to bombard He gas at 10-5-10-2 Torr. As electron bombardment favors the production of low states, it is possible to detect A transitions driven by microwaves. The microwave power was square wave modulated at 40 Hz, and the optical emission from a specific Rydberg state was monitored. When microwave transitions occurred to or... 

During the past five years two research disciplines of optical spectroscopy and magnetic resonance have merged when it became evident that at low temperatures, microwave radiation of resonance frequencies with the zero-field (zf) transitions of the lowest triplet state could have observable effects on the phosphorescence intensity as well as the spectrum. Quantitative information can then be obtained from these phosphorescence-microwave multiple-resonance experiments from which the magnetic, the radiative, and the nonradiative as well as the structural properties of the triplet state can be determined. 

Optical Detection of Electron-Nuclear Double Resonance (ENDOR) Transitions in Zero-Field... 

The earlier experiments done on optical detection of the Zeeman transitions were actually focused on determining the zero-field origin of the total intensity of the phosphorescence emission (15-17). The previous workers did not use a spectrometer as a part of their detection system. Fortunately most of the phosphorescence intensity of the systems studied by MODR originates from a single zf level. No optical spectroscopy has been done so far with the MODR methods in spite of the very important information that can be obtained from it. This is probably... 

Fig. 9. ODMR investigations at T = 1.4 K of Pd(2-thpy)2 dissolved in an n-octane Shpol skii matrix. Concentration = 10 mol/1 cw excitation Ag c = 330 nm (30.3 x 10 cm 0- Detection of the emission at 18418 cm (Tj —> Sq transition), (a) Zero-field ODMR (optically detected magnetic resonance) spectrum (b) Zero-field microwave recovery ODMR signal after pulsed microwave excitation with a microwave frequency of 2886 MHz. The best fit of the recovery signal is obtained with Eq. (4). (Compare Ref. [61])...

Fig. 8 (A) Electronic absorption spectra at room temperature of a doped PMM film, (B) MCD spectra at 1.8 K of a CHCI3 glass, and (C) MCD spectra at 1.8 K of a doped PMM film of [Mni20i2(02CCi5H29)i6]. In (B) and (C) applied magnetic fields are (-) -h 5T, (- -) - 5 T, (—) 0 T after application of -1- 5 T, ( ) 0 T after application of - 5 T. The sign of the retained CD spectrum in zero-field depends on the sign of the original applied field. The retention of magnetization depends on the polarization of the optical transition monitored. Figure from [101]...

The lifetimes of the sublevels of the excited triplet state of the Rh-trisdiimine complexes have been determined using the microwave recovery and adiabatic rapid passage techniques mentioned in Sect. 4.2. At (pumped) liquid helium temperatures it turned out that the triplet state sublevels have distinct lifetimes. As an example, we show in Fig. 8 the optically detected adiabatic transient signal as monitored for the zero-field D -1 resonance, at 2320 MHz, of the photo-excited [Rh(bpy)3] (0104)3 single crystal, at 1.4 K. The microwave frequency scan was at a rate of 2 x 10 Hz/s. Similar transients were obtained by rapid scans through the zero-field microwave transitions for the other compounds of the [Rh(phen)u(bpy)3 n] (0104)3 series. The transients fitted a biexponential function of the form... 

The lifetimes of the triplet-state sublevels of the mixed tris-cyclometalated Rh +-complexes, Rh(TTB)+, Rh(TPB)+, and Rh(PTB)+, were determined from optically detected microwave recovery and adiabatic rapid passage experiments performed at 1.4 K [66,75]. Typically, transients as displayed in Fig. 15 were obtained. The transients could be fitted in all cases to a bi-exponential of the form of Eq. (18). The ratio A/B equals the ratio of the radiative rate constants of the resonant sublevels. In Table 6 the resultant rate constants for the dopants Rh(TTB)+, Rh(TPB) and Rh(PTB)+ are collected. It is now obvious why for Rh(PTB)+ the 2 E zero-field ODMR transition could not be observed the and Ty sublevels have almost equal radiative probabilities. It is noted that the bi-... 

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