Intersystem crossing from singlet states of molecular dimers and monomers in mixed molecular crystals: picosecond stimulated photon echo experiments

An experimental study of the excited-state dynamics of pentacene dimers and monomers in p-terphenyl host crystals is presented. Picosecond stimulated photon echoes, picosecond photon echoes, and fluorescence lifetime measurements are used to study intersystem crossing and homogeneous dephasing of delocalized dimers and monomers at 1.8 K. It is found that the dimer states can have intersystem crossing rate constants which are orders of magnitude greater than the corresponding monomers. Three mechanisms are considered to explain the differences between dimer and monomer intersystem crossing. Fluorescence lifetime measurements and photon echo measurements demonstrate that the only source of homogeneous line broadening at 1.8 K is population relaxation. These measurements combined with the stimulated echo measurements show that differences in lifetimes exhibited by the various dimer and various monomer sites are due solely to differences in intersystem crossing rate constants.     

Picosecond time scale optical coherence experiments in mixed molecular crystals: pentacene in naphthalene

Picosecond time scale high-power pulse optical coherence measurements including photon echo and the stimulated photon echo are obtained with a mode-locked dye laser synchronously pumped by a double Q-switched and mode-locked Nd: YAG laser. Effects on coherence arising from excitation with gaussian laser pulses rather than square pulses are examined. Preliminary echo decay measurements are reported.     

Optical dephasing in a glass-like system: a photon echo study of pentacene in benzoic acid

Optical absorption and picosecond photon echo experiments are used to study the dephasing of pentacene in benzoic acid. It is shown that, while the absorption spectrum of pentacene is effected by proton transfer in the benzoic acid dimer, the dephasing is caused by elastic and inelastic phonon scattering processes.     

Temperature-dependent dephasing of delocalized dimer states of pentacene in p-terphenyl: picosecond photon echo experiments

Optical dephasing of pentacene dimers in p-terphenyl is studied. Dimer statesR₃ and R₄ exhibit an exponential temperature activation of T′₂ which is inconsistent with a mechanism involving scattering between delocalized dimer states. Other dephasing mechanisms are discussed. An upper limit is placed on the relaxation rate from the upper dimer state, R₁, to R₃     

Delocalized electronic excitations of pentacene dimers in a p-terphenyl host: picosecond photon echo experiments

Photon echo experiments are performed on delocalized electronic excitations of optical dimer states. The delocalized dimer states are found to have very long coherence times, close to the coherent limit The dimer dephasing is qualitatively similar to dephasing of pentacene monomers.     

Picosecond time-resolved cars in isotopically mixed crystals of benzene

Time-resolved coherent anti-Stokes Raman experiments are reported for the benzene crystal 991 cm^?1 mode and in mixed C₆H₆/C ₆D₆ crystals. Addition of C₆D₆ results in an increase in the rate at which the excitation is trapped. This is explained by the recent theory of Velsko and Hochstrasser which predicts that the trapping rate should undergo a maximum at some isotopic impurity concentration, consistent with observations from time and frequency domain studies. A proper convolution procedure for CARS experiments in the time domain is presented.     

Two-frequency spin echo in molecular crystals

The two-frequency pulse response of a multilevel system in NQR is investigated. Additional spin echo signals are shown to appear. The application of the two-frequency spin echo method to some of the crystals is demonstrated. The method is of great value for the investigation of local fields in crystals.     

Homogeneous optical dephasing and line broadening processes in an organic glass: comparison of the temperature dependences of picosecond photon echo and hole burning experiments

The homogeneous optical dephasing times (T₂) and the non-photochemical hole burning (NPHB) linewidths of resorufin in glycerol glass are reported from 1.1 to 25.5 K. At low temperature, the linewidths obtained from NPHB are substantially wider than the linewidths (1/πT₂) obtained from photon echo measurements, but the two become equal at high temperatures. The extra broadening of the NPHB is found to increase as T^0.8. A model of homogeneous dephasing including both two-level system and pseudolocal mode contributions is successful over the entire temperature range.     

Temperature-dependent vibrational dephasing in molecular crystals: a picosecond cars study of naphthalene

The temperature dependence of picosecond CARS of vibrational excitons in naphthalene is presented. Below ≈40 K relaxation is dominated by spontaneous emission of phonons. Above 40 K high-frequency phonons induce dephasing. The results are consistent with a phonon-promoted energy transfer process.     

Picosecond tri-transition and two-color photon echoes in a doped molecular solid

Picosecond tri-level photon echoes are generated among vibronic transitions of pentacene doped into a naphthalene host. The echoes are generated with three excitation pulses of which the first one, at ω₁, always excites a vibronic transition in the pentacene molecule. With the second excitation pulse at ω₂ and the third at ω₁, a tri-transition echo (TTE) is formed. With the time ordering of the second and third pulse reversed, a connected two-color stimulated echo (C2CSE) is generated. It is shown that, for small pulse angles, the low-temperature decay, of both echo effects is identical and that a smooth transition of one echo effect into the other occurs at the overlap in time between the second and third excitation pulse. Observation of these echoes further indicates that the inhomogeneous broadening at the selected transitions is strongly correlated.     

Cooperative effects in photon statistics of molecular dimers with spectral diffusion

The two-point fluorescence intensity correlation function g(2)t and the Mandel parameter Mt are calculated for a strongly pumped dimer of two-level molecules undergoing Gaussian-Markovian frequency fluctuations. The effects of detuning and saturation are examined. All fluctuation time scale regimes are explored using a continued fraction solution of the stochastic Liouville equation for the generating function. Bunching and antibunching are observed for slow and fast fluctuations, respectively. The short-time antibunching dip in g(2) and its variation with intermolecular coupling, the exciton annihilation rate, and laser detuning are studied.     

Energy trapping in mixed molecular crystals

We obtain numerically decay laws due to trapping on strongly-disordered two- and three-dimensional lattices, on which the migration proceeds through long-range dipolar steps. We find that the analytical decay laws for ordered lattices extend smoothly to disordered structures and that long-range steps override percolative effects.     

Photon-echo experiments on singlet-triplet transitions in organic mixed crystals

The homogeneous optical linewidth of a spin-forbidden S_o → T₁ transition in a chemically mixed organic crystal was directly measured in a two-pulse photon-echo experiment. The experimental values of T₂^Opt for the system dibromobenzophenone in dibromodiphenyl ether ranged from 1.4(3) to 3.8(2) μs, depending on the nuclear spins of the host. A temperature-dependent contribution to (T₂^Opt)^?1 revealed an activation energy ΔE of 13.8(5) cm^?1, which is tentatively attributed to a librational mode frequency.     

Study of polarisation ratios in mixed molecular crystals

The perturbation theory of Craig and Thirunamachandran is used to obtain the polarisation ratio (b/a) of guest transitions in an anthracene host crystal as a function of trap depth. Results from several substituted anthracenes as dopant are compared with the calculated curve. Substantial agreement is obtained over a wide range of trap depths confirming the applicability of the simple theory in this energy region. Degree of misorientation and possible deviation of transition moment from the short molecular axis (M) are briefly discussed.     

Photon emission statistics and photon tracking in single-molecule spectroscopy of molecular aggregates: dimers and trimers

Based on the generating function formalism, we investigate broadband photon statistics of emission for single dimers and trimers driven by a continuous monochromatic laser field. In particular, we study the first and second moments of the emission statistics, which are the fluorescence excitation line shape and Mandel's Q parameter. Numerical results for this line shape and the Q parameter versus laser frequency in the limit of long measurement times are obtained. We show that in the limit of small Rabi frequencies and laser frequencies close to resonance with one of the one-exciton states, the results for the line shape and Q parameter reduce to those of a two-level monomer. For laser frequencies halfway the transition frequency of a two-exciton state, the photon bunching effect associated with two-photon absorption processes is observed. This super-Poissonian peak is characterized in terms of the ratio between the two-photon absorption line shape and the underlying two-level monomer line shapes. Upon increasing the Rabi frequency, the Q parameter shows a transition from super- to sub- to super-Poissonian statistics. Results of broadband photon statistics are also discussed in the context of a transition (frequency) resolved photon detection scheme, photon tracking, which provides a greater insight in the different physical processes that occur in the multi-level systems.     

Picosecond dephasing times of HCl:Ar mixtures investigated by transient IR spectroscopy

A recent technique involving coherent propagation of infrared picosecond pulses is applied to various rotation-vibration transitions of HCl:Ar (R-branch). Ultrafast dephasing and beating of the two HCl isotopes is directly measured at several bar total pressure and compared with spectroscopic and theoretical data.     

Triplet electron spin dephasing in the disordered chemically mixed molecular crystal dichlorobenzene/dibromobenzene

Optically detected spin-locking experiments on the excited triplet state of p-dichlorobenzene (DCB) in p-dibromo-benzene (DBB) single crystals reveal at various DCB concentrations temperature-activated dephasing processes for the DCB monomers and one of the aggregates. The activation energies of 27 and 38 cm⁻¹ correspond to the energy separation of nearest-neighbour host molecules within induced energy funnels. Within the large concentration range 0.0001 to 0.06 mol/mol no influence of concentration on the spin dephasing could be observed.     

Characterization of tunneling systems in molecular versus polymer glasses by high-pressure photon echo spectroscopy

Intrinsic differences between tunneling two-level systems (TLSs) in molecular versus polymeric glasses are revealed by studying the effect of compression on TLS dynamics. Photon echo studies under variable low-temperature (1.1-2.3 K) and high-pressure (0-30 kbar) conditions have been performed to contrast the effect of compression on molecular [2-methyl-tetrahydrofuran (2MTHF)] versus polymer [Polymethylmethacrylate (PMMA)] glasses. The pressure-induced reduction in the magnitude of the optical dephasing rate of rhodamine 640 in a molecular glass (2MTHF) is found to be comparable to the volume decrease of the glass (e.g., approximately 20% at 30 kbar), indicating that TLSs in 2MTHF are associated with void space or low-density regions of the glass. In contrast, the relative pressure insensitivity observed for organic polymer glasses (PMMA) supports the idea that these TLSs are associated with side chain defects. The power-law exponent for the temperature-dependent dephasing in 2MTHF also decreased significantly at high pressure, suggesting a change in the form of the TLS density of states upon compression of the molecular glass.     

Detection of electronic and vibrational coherences in molecular systems by 2D electronic photon echo spectroscopy

Two-dimensional optical photon echo spectra are simulated for model systems which exhibit vibrational, electronic and a combination of electronic and vibrational coherent dynamics. The coherent motion manifests itself as periodic beatings of the spectrum cross-peak intensity with the population time. The intensity modulations are compared to evolution of the excited-state population and coordinate expectation value. The advantageous capabilities of the technique as well as possible difficulties in spectra interpretations are outlined. Possibilities for distinguishing electronic and vibrational coherences are discussed.