D. Huppert

Picosecond kinetics of p‐dimethylaminobenzonitrile

The nanosecond and picosecond resolved dual fluorescences of p‐dimethylaminobenzonitrile (DAB), in various solvents and glasses excited by 266 nm 20 ps FWHM laser pulses, have been investigated. Pulse‐limited rise times are exhibited by the b*‐state emission whose decay in turn feeds directly the risetime of a*‐state emission at 440–600 nm in most solvents studied. The a*‐state emission was monitored at 520–600 nm in order to eliminate contribution from the b*‐state. Within the experimental resolution, the b*‐state fluorescence decay times vary approximately linearly with solvent viscosity. The a*‐state fluorescence decay times vary with both solvent and temperature, and may reflect either thermally assisted intersystem crossing from the solvated singlet a*‐state (presumably of twisted internal charge transfer character) to a corresponding solvated triplet of slightly higher energy, or a thermally activated internal conversion of the 1TICT to the ground state.

Picosecond kinetics of chlorophyll and chlorophyll/quinone solutions in ethanol.

The mechanism of quenching by quinones of the lowest excited singlet state of chlorophyll has been investigated using picosecond laser spectroscopy. With chlorophyll alone, laser excitation resulted in immediate (less than 10 ps) bleaching of the 665 nm band and production of new absorption bands in the regions 460-550 and 800-830 nm. The lifetimes of these changes were greater than 500 ps. Addition of 2,6-dimethylbenzoquinone caused quenching of these absorbance changes. No indication of chlorophyll cation radical formation was obtained. Thus, the interaction between quinone and the chlorophyll excited singlet state results in energy dissipation without measurable formation of radical species having lifetimes longer than 10 ps. This is in marked contrast to the quenching of the chlorophyll lowest triplet state by quinones, during which easily detectable stable radical formation has been observed.

Picosecond kinetics of copper and silver protoporphyrins

By the use of the dual beam picosecond absorption spectroscopy technique, the initial events in the energy relaxation process of Cu(II) and Ag(II) protoporphyrin IX dimethyl esters were studied. The energy decay channels were found to be as follows. Cu(II) (Z=29) complex: After picosecond excitation, a Franck–Condon state of 2S1 is populated which decays into 2T1 within 8 psec. This state relaxes with a time constant of 450–460 psec to 2T1 and 4T1 equilibrium state from which phosphorescence is emitted. Ag(II) (Z=47) porphyrin: Picosecond excitation to the 2S1 state results in energy relaxation to 2T1 state within 8 psec which is followed by equilibration between 2T1 and 4T1 with a time constant of 11–12 psec. Subsequently the relaxation proceeds via nonradiative channels to 2Td and 4Td or 2(d,π*) states.

PICOSECOND AND NANOSECOND TSOMERIZATION KINETICS OF PROTONATED 11-CIS RETINYLIDENE SCHIFF BASES

Abstract— Picosecond and nonosecond spectroscopy has been used to study the isomerization mechanism of protonated 11‐cis retinylidene Schiff bases. The formation and bleaching of absorption bands within 10 ps and corresponding decay and recovery within 11 ns indicate that the isomerization mechanism of the protonated Schiff bases is not identical to rhodopsin in which the primary photophysical event is probably due to electron transfer or partial isomerization of the chromophore to a nonplanar conformation.

Radiationless decay of 1,3,5,7‐octatetraene

The decay kinetics of the 2 1Ag state of the linear polyene 1,3,5,7‐octatetraene in hydrocarbon solutions have been measured by nanosecond and picosecond techniques from 10 to 320 K. The observed decrease in emission lifetime with increasing temperature is well described in terms of a temperature activated intramolecular radiationless decay process proceeding over a ∼4 kcal Arrhenius barrier. This barrier is not significantly affected by intermolecular interactions such as sample viscosity or phase and may be related to rotation about essential single bonds in the excited state.

Intersystem crossing and predissociation of haloaromatics

The mechanism for the dissociation of several haloaromatic molecules has been investigated by means of absorption and emission picosecond spectroscopy. It is found that the rates of intersystem crossing and dissociation increase from chloro to bromo to iodo substituents. It is observed that the position of the halo substituent and temperature influence these rates. A model for the dissociation mechanism is proposed which is consistent with the absorption and emission data presented.

Effect of pressure on proton transfer rate in aqueous solutions: A picosecond study

Time resolved picosecond fluorescence studies of excited aqueous 8‐hydroxy‐1,3,6‐pyrene trisulfonate (HPTS) have been carried out at pressures up to the ice transition point of H2O and D2O. The proton transfer rates derived from these studies exhibit a large linear increase with pressure from 8×109 s−1 at 1 atm and 294 K to 2.5×1010 s−1 at the liquid→ice VI transition point at 9 kbar and 294 K. In D2O, the deuteron transfer rate also increases linearly with pressure from 2.7×109 s−1 at 1 atm to 1010 s−1 at 8 kbar. The rates between H2O and D2O were found to differ by a factor of 3, which is consistent with the isotope effect kD+/kH+=1/3 which is maintained across the pressure range studied. From these results, an activation volume ΔV‡0 of −6 cc/mol is obtained. These results which show different trends than previous NMR and proton conductance studies are discussed in terms of a microscopic model.

Subpicosecond relaxation of localized electrons in liquid ammonia

We report the results of an experimental study of the optical bleaching of the absorption spectrum of the solvated electron in dilute Na–NH3 and Na–ND3(2×10−4 M) solutions by a single 1.06 μ psec pulse (pulse duration 6 psec, pulse energy 30 mJ). The absorption spectrum of the solvated electron was monitored in the spectral range 1100–850 nm with the time resolution 20–3.3 psec. The line broadening of the solvated electron optical band originates from homogeneous broadening effects. The relaxation time of the excited state of the solvated electron was determined to be τ?2×10−13 sec. This ultrashort relaxation time favors relaxation between bound states of the solvated electron center.

A narrow‐bandwidth picosecond laser

We have examined the advantages of using Nd3+/YAG as an amplifier for a ∼6 psec FWHM pulse generated by a mode‐locked Nd3+/glass oscillator. The Nd3+/glass oscillator pulse is characterized by 100 cm−1 bandwidth at 1060 nm with variable temporal substructure. Passage of this pulse through a Nd3+/YAG rod resulted in wavelength‐selected amplification of the specific Nd3+/YAG lines located within the envelope of the Nd/glass pulse. The result is a 100‐fold amplified pulse consisting of two bands at 1061 nm and 1064 nm, each <3 cm−1 in bandwidth with an average duration of 6 psec and as low as 4 psec.

Picosecond dynamics of electron localization in metal/methyl amine solutions

Photoionization of the M− band of sodium/methyl amine by a picosecond pulse reveals the following: (1) The time of electron localization is less than 5 psec. (2) The bleaching of the solvated electron absorption band in the 1000 nm region with a relatively narrow bandwidth ∼100 cm−1 picosecond pulse at (1060 nm) is found to be proportional to the extinction coefficients. (3) The relaxation time of the excited state of the solvated electron was determined to be ∼2×10−13 sec.