Lifetime-Limited Photochemical Hole-Burning in s-Tetrazine-Benzene Mixed Crystals at 2 K

We show that the technique of photochemical hole-burning can be utilized to measure sub-nanosecond fluorescence lifetimes of molecular excited states. s-tetrazine is taken as an example and from the width of the hole (0.7 ± 0.1 GHz), burned in the origin of the lowest 1B3u ← 1A1g transition at 2 K in a benzene mixed crystal, a fluorescence lifetime of 455+75-55 ps is obtained. The coherence lifetime of the 16 a02 vibronic transition is found to be 25 ps

Fluorescence transient and optical free induction decay spectroscopy of pentacene in mixed crystals at 2 K. Determination of intersystem crossing and internal conversion rates

In this paper the recently reported fluorescence transient, appearing during the onset of cw dye laser excitation of the energetically lower sites of pentacene in p-terphenyl at 2 K, is analyzed quantitatively. The role of the lowest triplet state in causing the long (compared to the fluorescence lifetime) transient, is firmly established. For the lower sites a kinetic analysis yields a T1←S1 intersystem crossing (ISC) yield of about 0.4% and an S0 ←T1 ISC rate of (2.2 ± 0.1) × 10^4 s–1. Using these data in calculating the S1 ←S0 transition dipole moment from our earlier OFID experiments yields for the O1 site (proto) μ=0.71 ±0.24 D. The S0 ←S1 internal conversion yield is estimated to be 22%. The fluorescence transients of the energetically higher sites of pentacene in p-terphenyl exhibit a much faster decay, which indicates a drastic increase in ISC yield from S1. Optical free induction decay (OFID) experiments performed on these sites confirm this and imply that the T1 ←S1 ISC yield for these sites is ~60%. We further discuss results of an OFID experiment on pentecene in naphthalene and derive in this case at T1←S1 ISC yield of ≤2.8% and an S0←S1 internal conversion yield of 31.5 ± 1.5%. The off-resonance pumping effect in a 3-level system was also studied, using the parameter values for pentacene in p-terphenyl. For the lower sites, with the excitation intensities used (~1 MW/m2) the part of the inhomogeneous absorption line that is pumped has a width of about 2.5 GHz (effective excitation bandwidth <1 MHz).

Homogeneous Broadening of Optical Transitions in Organic Mixed Crystals

We have used the phenomenon of laser-induced molecular photodissociation to determine the homogeneous linewidth at 2 K of the origin (zero-phonon line) and a vibronic transition in the mixed-crystal absorption spectrum of dimethyl s-tetrazine in durene. From the measured 55-MHz (upper limit) homogeneous width of the origin we conclude that in the vibrationless excited state coherence persists at least during the 6-nsec lifetime. The 29-GHz homogeneous vibronic linewidth is ascribed to vibrational relaxation.

Photophysical and photochemical molecular hole burning theory

In this paper theories are given, describing photophysical and photochemical hole burning experiments on molecular mixed crystals at low temperature. Population saturation hole burning is treated for a two-level system where the lower level is the ground state. Hole burning due to a triplet state acting as a population bottle neck is also described by a steady-state density matrix theory. Connection is made with optical free induction decay. To obtain T2 (the decay time of the off-diagonal elements of the density matrix), which is the main goal of these hole burning experiments, a linear extrapolation method is discussed. For photochemical hole burning a time-dependent density matrix treatment is given. Using this theory numerical simulations were performed of the experimental results obtained by Völker and co-workers for porphin in n-octane at low temperature. Good agreement with experiment is obtained. For this case extrapolation methods are discussed in order to obtain reliable T2 values. A time-dependent kinetic theory is used to simulate the photochemical hole burning experiments on dimethyl-s-tetrazine in durene and s-tetrazine in benzene. This theory accounts for the recently revealed two-photon character of these photodissociations. It is shown that despite this complication the hole full width at half-maximum may still equal twice the homogeneous width of the S0-S1 transition.

Numerical simulations of accumulated stimulated photon echoes

A theory has been developed describing probe pulse-detected accumulated stimulated photon echo experiments on inhomogeneously broadened electronic transitions. This theory gives a more quantitative description of these experiments than an earlier developed theory. At the same time, the present theory provides numerical instead of analytical results. A better founded insight is obtained in the possibilities and limitations of the accumulated photon echo method. The main conclusion is that the measured echo decay curves are still expected to give T2 directly, thus maintaining the value of the accumulated echo technique as a convenient method to determine picosecond dephasing times. It is concluded that the probe pulse detected accumulated echo method is not readily applied to accurate determinations of intersystem crossing yields.

Optical Free Induction and Fluorescence Decay of Pentacene in p-Terphenyl Following Narrow Band Laser Excitation

Optical free induction and fluorescence decay experiments on pentacene following narrow band laser excitation are reported. From the results we conclude that pentacene for its lowest 1B2u ← 1A1g transition is a perfect two-level system and from the power saturation term in the OFID we calculate a transition dipole moment of 1.5 ± 0.2 debye. A long-lived transient observed in the fluorescence during the onset of cw dye laser excitation is ascribed to a bottle-neck in the optical pumping cycle.

Optical free induction decay of molecules undergoing radiative and radiationless relaxation

In this paper we examine the effect of radiative and radiationless processes on optical free induction decay (OFID) in singlet–singlet transitions of molecules in the condensed phase. The main conclusion is that the applicability of OFID to molecular singlet–singlet transitions is severely limited by the presence of an intermediate triplet state, which quite often forms a bottleneck in the optical pumping cycle. Using the theoretically derived formulas for the OFID signal, we examine the feasibility of OFID experiments on the mixed crystals of porphin in n-octane and naphthalene in durene and conclude that such experiments, if at all fruitful, would be difficult to execute. We re-evaluate the reported OFID results of pentacene in p-terphenyl and obtain for the lowest 1B2u ← 1A1g transition a dipole moment of 0.9 ± 0.3 D.