A New Approach to the Vibronic Spectra of Molecular Crystals

Abstract

A st.udy is made of the experimental consequences of the theory of the vibrational spectra of molecular crystals presented recently in L3]. A discussion is also given of the model used irz this theory. A detailed comparison is made between t.hc theory and experimental resultv for pure and isotropic doped napht.halene crystals in the M-band region. The theory accounts for the decay of electronic-vibrational intra-molecular excitations into pure electronic and intra-molecular vibrational excitations by a resonance interaction. I n this case the crystal spectrum should exhibit a wide absorption band corresponding to two-particle excitation. If the reduction of the vibrational frequencies of the molecule accompanying its electronic excitation is comparable with the pure electronic excitat.ion band width, the spectrum should also show a narrow band corresponding to a one-particle excitation. These bands should be associated with the vibrational band which is observed experimentally. This idea is used to analysis the absorption in t.he first vibrational transition region of a naphthalene crystal. The one-and two-particle excitation regions are identifird and B quantitative analysis of the data is made which is in accordance with the theory. Collect.ive effects, connected with the excitation decay, appear to be important in this transition region. In this connection of particular interest is an analysis of peculiarities of impurity vibrational absorption in isotopic solutions. Comparison between the spectral dat,a naphtalene deutero-isotopes and the theory also indicates the importance of some complicated collective interactions. ann E e T a n 1 , H o r o a r i a J r a a a I I o r a o q e m R R o 6 n a c~~ r~epnoro na6p01111oro Iiepexona H p a c T a m a H a @ T a m i r a , 6bIm B b i A e n e I m 06nac~n o~o -13 X n y x q a c T m a b I x ~0 3 -6 y ) I i Z C H H P H r I p o i , e ; I e I I a 0 6 p a 6 O T I i a j a r r m I x R COOTBCTCTBAA c T C O~A C~~. O B I r a p y -

Remarks on Exciton—Phonon Coupling and Exciton Transport

“…If one chooses to work in the site representation, Eq. (14) shows that the a,,(oo) are complicated functions which are in general nonzero for n # m. Transformation of Eq. (7) to this representation shows that the time derivatives of the diagonal elements a , , are in general coupled to the off-diagonal elements anm , so that a master equation like Eq.…”

Transport of Quasilocalized Excitons in Molecular Crystals

“…Th~ where the subscript A labels the molecular normal modes. The condition (69) is less strict than its counterpart (28), and could admit changes in the total number of phonons if the WA are not all close in value, However, at moderate temperatures transitions in which the total number of phonons does not change will predominate, At this point the simple classification of limiting cases adopted earlier must be modified, since the phonons belonging to different normal modes will in general have different velocities and different strenaths of interaction with the exciton. One can still disting~ish coherent and incoherent modes of transport, using the same mean-free-path criterion as before.…”

“…38 When the oscillators are distorted but not displaced, the overlap can be written as (28) Provided that W2 is not too large a fraction of Wo, so that wand Wo are close in value, and the temperature is not so high that the Vn are large, this equation can only be satisfied if both sides are zero. In molecular crystals these conditions should be fulfilled: w is unlikely to exceed about 0.2wo, and because of the weak intermolecular forces the crystal is likely to melt at temperatures such that the average value of the Vn is only 1 or 2.…”

“…In molecular crystals these conditions should be fulfilled: w is unlikely to exceed about 0.2wo, and because of the weak intermolecular forces the crystal is likely to melt at temperatures such that the average value of the Vn is only 1 or 2. Equation (28) thus implies that simultaneously Vn = Vn+l' and v,.' = Vn+l, so that the phonons originally at sites nand n+ 1 must interchange sites as the exciton jumps between the sites.…”

Polariton effects in naphthalene crystals

“…Strong and weak coupling models (101)(102)(103)(104)(105)(106)(107)(108)(109) for Frenkel excitons and phonons have been discussed at length. For higher vibronic transitions in crystals such as naphthalene the concept of two-particle states (states in which vibrational and electronic excitation are located on different sites) has proven useful in interpreting spectral data (110)(111)(112)(113)(114)(115)(116)(117).…”