Zero-phonon hole

An experimental PHB hole profile generally consists of three parts a sharp zero-phonon hole, a small, broad hole of the phonon side band on the higher energy side, and a broad hole, called pseudo-phonon side hole, on the lower energy side of the laser frequency (Table 2.12 ). The pseudo-phonon side hole results from the overlap of the zero-phonon holes which exhibit phonon side bands at the laser frequency. 

Phonon frequency, E. (cm ), defined as energy difference between the zero-phonon hole and the pseudophone sidehole and the energy for the low-energy excitation mode obtained from heat capacity measurements, E. (cm ... 

This phenomenon comes from the fact that the spectrum of the hole is actually a convolution of the single molecule line shape (Fig. lb) with itself, reflecting the two procedures associated with the registration of a hole burning spectrum, namely, burning and reading. That the overall hole shape is asymmetric with respect to the zero-phonon hole is a saturation phenomenon. It should also be stressed that, because of this convolution procedure, the width of the zero-phonon hole Fh is actually twice the homogeneous line width ... 

Fig. 8. Low-temperature absorption (4 K) and 40D spectra of phycobilisomes of Masti-gocladus laminosus. Laser excitation was carried out at 6380 A, as documented by the sharp zero-phonon hole [From W. Kohler, J. Friedrich, R. Fischer, and H. Scheer, J. Chem. Phys. 89, 871 (1988)].

Figure 10 shows a hole burnt into the perdeuterated reaction center of wild-type Rhodobacter sphaeroides. There are some noteworthy features. First, there is a rather sharp, although small, zero-phonon hole with a width of 6 cm. According to Eqs. (3) and (4), the associated lifetime is... 

Fig. 10. AOD spectrum of the deuterated reaction center P870 of wild-type Rhodobacter sphaeroides. Note the sharp zero-phonon hole at the laser wavelength 910 A [Data from N. R. S. Reddy, P. A. Lyle, and G. J. Small, Photosymh. Res. 31, 167 (1992)].

Second, apart from the sharp zero-phonon hole in Fig. 10, there is a broad side hole whose maximum is shifted to the blue compared to the laser frequency. The side hole represents molecular and lattice vibrational transitions which are simultaneously bleached with the pure electronic line. From the relative magnitude of the area under the zero-phonon hole and the side hole, we see that the coupling to vibrational transitions is rather strong, as is expected for states with a considerable amount of charge transfer character. 

Figure 11 shows a saturated hole proffle burned to estimate the low-energy excitation mode, E of TPP/PnPMA. A deep zero-phonon hole is created at 644.8 nm. In addition, there are two broad holes at bodi sides of the zero-phonon hole. The broad hole at the shorter wavelength is called a phonon side hole and the one at the longer wavelength is called a pseudo-phonon side hole . The phonon side hole consists of a phonon side band of the zero-phonon hole, and the p udo-phonon side hole is made of the zero-phonon line of the reacted molecules which have been excited via phonon side band. 

A persistent spectral zero-phonon hole was obtained at liquid helium temperatures by Kr laser light irradiation (520.8 nm). In spite of the high concentration of DAQ (ca. 1.5 mol kg ), a narrow hole with an initial width of 0.25 cm (4.6 K) was obtained. This width is narrower than those (e.g., 0.4-0.8 cm ) of DAQ doped in ordinary polymers and organic glasses (e.g., PMMA, ethanol/methanol mixed glass) obtained under similar experimental conditions. The narrow line width is desirable for the application to optical recording, since... 

At room temperature, non-photochemical spectral holes usually are filled in by flucmations of the surroundings on the picosecond time scale. This process, termed spectral dijfusion, can be studied by picosecond pump-probe techniques. At temperatures below 4 K, non-photochemical spectral holes can persist almost indefinitely and can be measured with a conventional spectrophotometer. The shape of the hole depends on the lifetime of the excited state and the coupling of the electronic excitation to vibrational modes of the solvent, both of which depend in turn on the excitation wavelength. Excitation on the far-red edge of the absorption band populates mainly the lowest vibrational level of the excited state, which has a relatively long lifetime, and the resulting zero-phonon hole is correspondingly sharp (Fig. 4.22A). The zero-phonon hole typically is accompanied by one or more phonon side bands that reflect vibrational excitation of the solvent in concert with electronic excitation of the chromophore. The side bands are broader than the zero-... 

In similar studies of photosynthetic reaction centers, the width of the zero-phonon hole for the reactive bacteriochlorophyll dimer was related to the time constant for electron transfer to a neighboring molecule [56, 67, 68]. Figure 4.23 shows a typical hole spectrum (the difference between absorption spectra measured with the excitation laser on and off) for a sample of reaction centers that was excited at 10,912 cm at 5 K. The holes in this experiment resulted from the... 

Fig. 4. 23 The spectrum of a photochemical hole burned in the long-waveloigth absorption band of a sample of photosynthetic bacterial reaction centers at 5 K [68]. The gray curve is the difference between absorption spectra measured with the excitation laser on and off. The excitation frequency was 10,912 cm. Note the sharp zero-phonon hole PH, upward arrow) at 10,980 cm . The downward arrows indicate the centers of two discrete vibrational (phonon) bands that are linked to the zero-phonon transition. The solid curve is a theoretical hole spectrum calculated as described in the text...

Relaxations of solvent-chromophore interactions can be studied experimentally by hole-burning spectroscopy, time-resolved pump-probe measurements, and photon-echo techniques that we discuss in the next chapter. If the temperature is low enough to freeze out pure dephasing, and a spectrally narrow laser is used to bum a hole in the absorption spectmm (Sect. 4.11), the zero-phonon hole should have the Lorentzian lineshape determined by the homogeneous lifetime of the excited state. The hole width increases with increasing temperature as the pure dephasing associated with tP comes into play [36, 37]. 

Fig. 1. PS II reaction center low temperature spectra. Top 4.2 K absorption spectrum of preparation A (dashed line) and B (solid line). Optical density at P680 maximum is 0.2 and 0.5, respectively. Bottom persistent hole burned spectrum of preparation A (dashed line) and preparation B (solid), 4.2 K. Bum wavelength (A. ) for each spectrum is coincident with the sharp zero-phonon hole near 665 nm. The broad ( 120 cm" ) Pheo a satellite holes at 681.6 nm (preparation A) and 681.3 nm (preparation B) are due to energy transfer from the Chi a pigments excited at Xg. The Pheo a holes represent a peak percent absorbance change of 7%. Hole burning conditions Ig = 200 mW/cm Tg = 20 min Tg = 4.2 K...

The first term describes the so-called zero phonon hole (ZPH). Its halfwidth is the double homogeneous halfwidth of ZPL. The second and third terms describe the so-called phonon side hole (PSH). However, in contrast to PSB from FLN spectra the PSH is shifted to the red and blue sides with respect to ZPH (Fig. 14a). The fourth term describes a structureless phon whidi can be ignored if we are interest in the PSB shape. However, the jdion term should be taken into account if we determine the Debye-Waller factor with the help trf HB spectra. This situation resembles that existing in FLN spectra. 

Fig. 14a, b. Zero-phonon hole and phonon-side holes at a small and b large intensities of burning light... 

It has b n shown in Sect 4.3 that the halfwidth of zero-phonon holes equals the double homogeneous halfwidth of ZPL. However, this statement requites correction b use it does not allow for the efto d SD and the possitnlity that chromophores with a single resonant frequency csm have ZPLs with different homogeneous halfwidths. However, if the distribution over homogeneous halfwidths is of importance the hole has a non-Lorentdan shape. Experimental data reveal the Lorentzian hole shape [38]. Therefore we can neglect this distribution and take the theoretical expression for hole hallwidth as a sum of three terms ... 

In previous work it has been shown that the width of the zero-phonon hole (ZPH) in the photochemical hole burned spectrum of the P-band can be used to determine the lifetime of P from its total zero-point level which, incidentally, is a determination not possible by ultra-fast spectroscopy. 

Figure 2. Photochemical hole burned spectra of P870, T= 4.2 K. From top to bottom the bum frequencies are 10921, 10957, 10992 and 11039 cm". The arrows locate the zero-phonon holes, each of which is coincident with 3. The bum intensity is the same for all spectra, 10 mW/cm. From top to bottom the %-absorbance changes (as measured at the maximum of the broad hole) are 6.1%, 8.7%, 20.6 and 22.6%.

Figure 3. Lorentzian fits (dashed profiles) to the zero-phonon holes of Figure 2. The curves are normalized to the largest delta absorbance. 

The observed transient absorption data alone are not able to eliminate Model B2. Additional information comes from hole-burning experiments (Johnson et al., [22]). In these experiments performed at very low temperatures narrow zero phonon holes were observed with a spectral width corresponding to a time constant of approximately 1 ps. From these data one can deduce that the first reaction process starting from the lowest vibrational level of P is the slower, the 1.4 ps process. The faster 0.3 ps component must be (as it is not related with vibrational relaxation, see above) the second process in the reaction scheme. Since the important features of the reaction processes do not change strongly with temperature one may discard Model B2 at room temperature as well. 

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