Hole-burning

Molecules in crystals or dispersed in host lattices are often present in a range of environments, and this results in a broadening of the electronic absorption spectrum. Such an inhomogeneously broadened absorption band (envelope of transitions) may be considered as a superposition of several distinguishable sites. A narrow line laser can saturate one of the transitions under the envelope and the corresponding molecules will no longer take part in the absorption process. This phenomenon is referred to as hole 

Due to saturation, the population density A i(Uj)dv2 decreases within the velocity interval dv = y/k, while the population density N2ivz)dv of the upper level 2) increases correspondingly (Fig. 2.4a). From (2.10) and Vol. 1, (3.72) we obtain for 5 1 

Subtracting (2.24b) from (2.24a) yields for the saturated population difference 

For 5o = 1 the hole depth amounts to 50 % of the unsaturated population difference. Molecules with velocity components in the interval to + du give the contribution 

Doppler-broadened absorption profile a (o) burned by a strong pump laser at cop = m and detected by a weak probe laser at 0) = 0)0 (o)i — coo)ki /k2 

29) illustrates a remarkable result Although at each frequency co the monochromatic laser burns a Bennet hole into the velocity distribution N vf)y this hole cannot be detected just by tuning the laser through the absorption profile. The absorption coefficient 

Note that for y / K2 the depth of the hole in N (Vz) and the heights of the peak in N2(Vz) are different. Subtracting (7.18b) from (7.18a) yields for the saturated population difference 

The velocity-selective minimum in the velocity distribution AN(vz) at i , = co — coo)/k, which is often called a Bennet hole [7.1], has the homogeneous width (according to Sect. 3.6) 

When a monochromatic light wave E = Eocos(o t - kz) with k = 

Molecules with velocity components in the interval v to v +dv give the contribution 

Note the difference to the homogeneous absorption profile where a(w) is reduced by the frequency-dependent factor 1+S(w), see (3.88) and Fig. 3.24. 

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Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If tlie pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112 and 113] can be perfomied. The second type of experiment, dynamic absorption spectroscopy [57, 114. 115. 116. 117. 118. 119. 120. 121 and 122], can be perfomied if the pump and probe pulses are short compared to tlie period of the vibrational modes that are coupled to the electronic transition. 

Figure B2.1.6 Femtosecond spectrometer for transient hole-burning spectroscopy with a continuum probe. Symbols used bs, 10% reflecting beamsplitter p, polarizer. The continuum generator consists of a focusing lens, a cell containing flowing water or ethylene glycol or, alternatively, a sapphire crystal and a recollimating lens.

Figure B2.1.7 Transient hole-burned speetra obtained at room temperature with a tetrapyrrole-eontaining light-harvesting protein subunit, the a subunit of C-phyeoeyanin. Top fluoreseenee and absorption speetra of the sample superimposed with die speetnuu of the 80 fs pump pulses used in the experiment, whieh were obtained from an amplified CPM dye laser operating at 620 mn. Bottom absorption-diflferenee speetra obtained at a series of probe time delays.

In some extremely iimovative recent experiments, Hochstrasser and co-workers [ ] have described IR transient hole-burning experiments focused on characterizing inliomogeneous broadening in the amide 1... 

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Zilker S J, Kador L, Friebel J, Vainer Y G, Kol chenko M A and Personov R I 1998 Comparison of photon echo, hole burning, and single molecule spectroscopy data on low-temperature dynamics of organic amorphous solids J. Phys. Chem 109 6780-90... 

Fig. 22. Photochemical hole burning (PHB) (1,173) where CO is frequency CO, frequency of the laser and CO p, frequency of the photoproduct.

Applications Involving Nonlinear Absorption Phenomena. Saturable absorption (hole-burning) is a change (typically a decrease) in absorption coefficient which is proportional to pump intensity. For a simple two level system, this can be expressed as... 

LB Films of Porphyrins and Phthalocyanines. The porphyrin is one of the most important among biomolecules. The most stable synthetic porphyrin is 5,10,15,20-tetraphenylporphyrin (TPP). Many porphyrin and phthalocyanine (PC) derivatives form good LB films. Both these molecules are important for appHcations such as hole-burning that may allow information storage using multiple frequency devices. In 1937 multilayers were built from chlorophyll (35). 

W. E. Moemer, ed.. Persistent Spectral Hole Burning Science andMpplications, Spriuger-Vedag, Heidelberg, Germany, 1988. 

Tliere are several reasons for this great interest in the tautomerism of porphyrins (which could justify its own review) (1) their biological significance, (2) their applications in material science ( hole burning is related to their tautomerism), (3) the simplicity of the system (annular tautomerism involving intramolecular proton transfer both in solution and in the solid state), and (4) the possibility of elucidating the kinetic processes in great detail. 

It is well-known that para substituents on the phenyl groups of H2TPP have no influence on the tautomerism rates in the ground state (see Section III,A,1). In the case of PHB, there seems to be only a small substituent effect on <1>phb (the quantum efficiency for hole burning) through modification of the relative energy of Ti (93CM366). 

Friedrieh has studied the free-base eytoehrome c in a glass state at 1.6 K (96MI77). In general, for free-base protoporphyrins it ean be safely assumed that the hole-burning photoreaetion is based on a light-indueed rearrangement of the inner protons. This type of reaetions has been verified for all free-base porphyrin moleeules [92MI(61)381]. 

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