Fluorescence spontaneous

Stanley R J and Boxer S G 1995 Oscillations in the spontaneous fluorescence from photosynthetic reaction centers J. Phys. Chem. 99 859-63... 

The vast majority of single-molecule optical experiments employ one-photon excited spontaneous fluorescence as the spectroscopic observable because of its relative simplicity and inlierently high sensitivity. Many molecules fluoresce with quantum yields near unity, and spontaneous fluorescence lifetimes for chromophores with large oscillator strengths are a few nanoseconds, implying that with a sufficiently intense excitation source a single... 

With normal efforts for frequency stabilization in the optical frequency region, time-averaged laser linewidths of about 1 Mc/sec have been obtained 3 ), compared to 1000 Mc/sec in case of spontaneous fluorescence lines. 

Using as the background continuum the short-lived spontaneous fluorescence of rhodamine B or 6 G, McLaren and Stoicheff 233) developed this method further to obtain inverse Raman spectra over the range of frequency shifts 300-3500 cm" in liquids and solids in a time of 40 nsec The stimulating monochromatic radiation at 6940 A is provided by a giant-pulse ruby laser. A small part of the main laser beam is frequency-doubled in a KDP-crystal and serves to excite the rhodamine fluorescence, thus ensuring simultaneous irradiation of the sample by both beams. 

The spontaneous fluorescent decay time tf is connected with the radiative lifetime tfr and the quantum yield of fluorescence ijf by jjf =tf/tfr. Since the radiative lifetime is of the order of a few nanoseconds in most dyes, the spontaneous fluorescent decay time is about the same for quantum yields of fluorescence near unity (i.e., k Q ttksT 0) and decreases to a few picoseconds for quantum yields of the order of 10-3. 

This effective dye relaxation time rp is the spontaneous fluorescence decay time shortened by stimulated emission which is more severe the higher the excitation and therefore the higher the population density w j. The dependence of fluorescence decay time on excitation intensity was shown in 34 35>. Thus, fluorescence decay times measured with high intensity laser excitation 3e>37> are often not the true molecular constants of the spontaneous emission rate which can only be measured under low excitation conditions. At the short time scale of modelocking the reorientation of the solvent cage after absorption has occurred plays a certain role 8 > as well as the rotational reorientation of the dye molecules 3M°)... 

Jablonski" diagram, showing, for a molecule in the ground (spin)-singlet state S0, the (induced) absorptions, a double-quantum transition, (spontaneous) fluorescence, (spontaneous) phosphorescence, internal conversion, and intersystem crossing between the singlet manifold of states S0, S1 S2, and S3, and the lowest excited triplet state T-. ... 

An unusually good energy match is required before these intersystem relaxation paths compete with more rapid processes such as spin-relaxation within a multiplet state, vibrational relaxation, or spontaneous fluorescence (NO B—>X). It is interesting that the B2IIi/2 —> a4n5/2 ACl = 2 process seems to require a better energy match than ACl = 1 or 0 processes even though the quantum numbers Cl, A, and E are destroyed by the noncylindrical matrix site. 

As discussed in Section 8.2, superexcited states, AB, can decay by both autoionization and dissociation (more specifically, by predissociation). Decay by spontaneous fluorescence can be neglected for superexcited states because, generally, the predissociation or autoionization rates (l/rnr 1012 to 1014s-1) are much faster than the fluorescence rate (l/rr < 108s-1). Only two examples of detected spontaneous fluorescence from superexcited states have been reported (for H2, Glass-Maujean, et ai, 1987, for Li2, Chu and Wu, 1988). The H2 D1 e-symmetry component is predissociated by an L-uncoupling interaction with the B 1B+ state (see Section 7.9 and Fig. 7.27). Since a 4E+ state has no /-symmetry levels, the /-components of the D1 A-doublets cannot interact with the B E+ state and are not predissociated. The v = 8 level of the D1 state, which lies just above the H/ X2E+ v+ = 0 ionization threshold, could in principle be autoionized (both e and / components) by the X2E+ v+ = 0 en continuum. However, the Av = 1 propensity rule for vibrational autoionization implies that the v = 8 level will be only weakly autoionized. Consequently, the nonradiative decay rate, 1 /rnr, is slow only for the /-symmetry component of the D1 v = 8 state. Thus, in the LIF spectrum of the D1] —... 

Similar formulations of spontaneous fluorescence, stimulated emission pumping, and Raman spectra as the (full- or half-) Fourier transform of an autocorrelation exist (Tannor, 2003) and provide unique insights into the forces responsible for the early-time dances of atoms subsequent to an artfully designed and implemented initially localized excitation. 

The fundamental idea is that the pump and probe pulses create wavepackets, which evolve on the excited state potential surface. Interference between the excited state wavefunction amplitudes created by the two pulses affects the population transferred to the excited state. The population that is measureable in a typical incoherent experiment (spontaneous fluorescence, field ionization, excitation to a different excited state by a nanosecond pulsed laser) is proportional... 

In particular, the laboratory frame orientation of the transition moment for spontaneous fluorescence evolves in time. The intensities of z— and (x,y) — polarized fluorescence are modulated 7t/2 out of phase, but the intensity of the total x + y + z polarized fluorescence is not modulated. This is the physical basis for polarization quantum beats (Aleksandrov, 1964 Dodd, et al., 1964) and Rotational Coherence Spectroscopy (Felker and Zewail, 1995). 

In the phase-coherent, one-color pump/probe scheme (see Section 9.1.9) the wavepacket is detected when the center of the wavepacket returns to its to position, (x)to+nT — (x)to, after an integer number of vibrational periods. The pump pulse creates the wavepacket. The probe pulse creates another identical wavepacket, which may add constructively or destructively to all or part of the original pump-produced wavepacket. If the envelope delay and optical phase of the probe pulse (Albrecht, et al, 1999) are both chosen correctly, near perfect constructive or destructive interference occurs and the total spontaneous fluorescence intensity (detected after the pump and probe pulses have traversed the sample) is either quadrupled (relative to that produced by the pump pulse alone) or nulled. As discussed in Section 9.1.9, the probe pulse is delayed, relative to the pump pulse, in discrete steps of At = x/ojl- 10l is selected by the experimentalist from within the range (ljl) 1/At (At is the temporal FWHM of the pulse) to define the optical phase of the probe pulse relative to that of the pump pulse and the average excitation frequency. However, [(E) — Ev ]/K is selected by the molecule in accord with the classical Franck-Condon principle (Tellinghuisen, 1984), also within the (ojl) 1/At range. When the envelope delay is chosen so that the probe pulse arrives simultaneously with the return of the center of the vibrational wavepacket to its position at to, a relative maximum (optical phase at ojl delayed by 2mr) or minimum (optical phase at u>l delayed by (2n + l)7r) in the fluorescence intensity is observed. 

Rotational recurrences may be detected in polarization selected spontaneous fluorescence (provided the photodetector has a sufficiently fast response) or by a variety of sub-nanosecond pump/probe schemes (Felker and Zewail, 1987 Felker, 1992 Hartland, et al. 1992 Joireman, et al.. 1992 Smith, et al., 2003a,b). 

Crystal Active center Concentration of active centers (cm ) Spontaneous fluorescence lifetime Wavelength... 

We excite the molecule from the ground vibrational state of G to a certain vibrational state of M using a laser. Then the molecule undergoes a spontaneous fluorescence transition to REP. The electronic state changes so fast that the nuclei have no time to move (the... 

Finally in figure 13c we depict the scheme for stimulated emission pumping, which is the stimulated equivalent of dispersed LIF. I Jhen the probe is on resonance this stimulates emission in the direction of the probe laser beam, thereby depleting spontaneous fluorescence. The resonance condition can therefore be sought through detection of fluorescence side-light without the losses inherent in the use of a monochromator. 

The propogation of an intense, nearly resonant, laser pulse, through sodium vapor results in a number of nonlinear scattering processes. These processes have been previously observed in the spontaneous fluorescence Spectra emitted by Na vapor interacting with a laser tuned near the D lines.( ) Here we report the observation of stimulated emission due to the same processes which we obtain whenever the appropriate population differences exist between the states involved in the scattering.(2)... 

Figure 10.7 Normalized fluorescence (a), excitation (b) and optically pumped PL spectra (c, d) taken from platelet crystals of BP1T. The spontaneous fluorescence (a), AST (c) and SRRS (d) spectra were taken at Xex = 365, 355 and 460 nm, respectively. The excitation spectrum (b) was taken for the fluorescence band at 493 nm. Reproduced from H. Yanagi, I. Sakata, A. Yoshiki, 5. Hotta and 5. Kobayashi, Polarization dependence of Stimulated resonance Raman scattering from a single crystal of bi-phenyl-capped thiophene, Jpn. J. Appl. Phys., 45, 483-487 (2006) with permission from The Institute of Pure and Applied Physics...

The method of single photon counting [201], [202J is generally used for such lifetime determinations. A weak pulsed light source causes excitation of a small number of molecules. Spread over a period of time (spontaneous fluorescence is a statistical process), single fluorescence photons... 

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