Frequency modulation, single molecule

The first SMS experiments in 1989 utilized either of two powerful doublemodulation FM absorption techniques, laser frequency-modulation with Stark secondary modulation (FM-Stark) or frequency-modulation with ultrasonic strain secondary modulation (FM-US) [3,26]. The secondary modulation was required in order to remove the effects of residual amplitude modulation produced by the imperfect phase modulator. In contrast to fluorescence methods, Rayleigh and Raman scattering were unimportant. Figure 2.3B (specifically trace d) shows examples of the optical absorption spectrum from a single molecule of pentacene in p-terphenyl using the FM-Stark method. 

To conclude, the second dimension is introduced if the switching time ti (Fig. 2.48) is incremented in a series of single experiments so as to reach all possible double quantum frequencies vDQ within a sample molecule by the reciprocals l/t1. Again, the acquired FID signals will depend on two variable times t1 and t2, respectively. A first Fourier transformation in the t2 domain generates 13C — 13C satellite spectra. The corresponding AB or AX type doublet pairs, however, are modulated by the individual double quantum frequencies which characterize each AB or AX pair. The second Fourier transformation in the tl domain liberates the double quantum frequency as the second dimension Maximum AB or AX 13C—13C subspectra are observed at the corresponding double quantum frequencies, so that each doublet appears with unique coordinates,... 

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). 

Fig. 1.16 (A) Fluorescence (so/id curve) from a molecule that is excited with sinusoidally modulated light (dotted curve). If the fluorescence decays exponentially with single time constant T, the phase shift (4>) and the relative modulation of the fluorescence amplitude (w) are related to t and the angular frequency of the modulation m) by < = arctan((OT) and w = (1 + o> ) (Appendix A4). The curves shown here are calculated for t = 8 ns, cd = 1.257 x 10 rad/s (20 MHz) and 100% modulation of the excitation light (< = 0.788 rad, i = 0.705). (B) Phase shift (4>, solid curve) and relative modulation (m, dotted curve) of the fluorescence of a molecule that decays with a single exponential time constant, plotted as a functirai of the product on. The relationships among , m, r and m become more complicated if the fluOTescence decays with multiexponential kinetics (Appendix A4)...

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