Search

Single crystals of NaCl containing Na 2 S or NaOH exhibited an absorption band at 1.18 eV under illumination of light absorbed by the F band at room temperature after additive coloration. Further illumination at 80 K changed the peak position from 1.18 eV to 1.14 eV. The bands arise from both F 2 + :S - and F 2 + :O - centers, in which the F 2 + center is loosely bound by a S - or O - ion (1.18 eV:type A) or most compactly bound (1.14 eV:type B). Keeping the sample at a desired temperature between 205 and 220 K resulted in recovery of the peak position to 1.18 eV. Energy for recovery was estimated to be 0.50±0.01 eV and 0.54±0.02 eV for F 2 + :S - and F 2 + :O - centers, respectively. Comparison of the energies with that for rotation of the dipole axis of the F + +O - pair, or for migration of an anion vacancy suggests that 60 degree rotation of the F 2 + axis in the F 2 + :S - or F 2 + :O - center causes restructuring of the center from type B to A.

The picosecond spectroscopy with the time delayed double excitation technique has revealed that the F - H center pair ([ F - H ]) is converted from the self-trapped exciton ( S T E L ) at the a 1 g state within 20 ps in KCl, NaCl and RbCl at 14 K. Conversion efficiencies from the S T E L to the [ F - H ] and to unknown X states were estimated by assuming two possible candidates of the hole π g state or the higher electronic state in the STE as the precursor for [ F - H ] formation. Percentages of branching from the precursor to the [ F - H ] were 49, 86 and 20% in KCl, NaCl and RbCl, respectively. Rest of the STE were converted to unknown X states. By making use of these results, we criticized the state acceptable as the precursor, i.e., the hole π g state or the electronic states.

Time delayed double excitation spectroscopy in the picosecond range has revealed transient F bands due to the F center perturbed by an H center at the next nearest site, [ F - H ] n n n , in RbCl, KCl and NaCl single crystals after excitation of the self-trapped exciton (STE) from the lowest triplet state to higher excited states. The F band consists of a doublet peaking at 1.99 and 2.11 eV (RbCl), at 2.25 and 2.38 eV (KCl) and at 2.70 and 2.86 eV (NaCl) with the ratio of 2:1 in height in every crystal at 108 ps after the excitation. The peak positions and relative height of the doublets are understood qualitatively by the theory for the off-center type S T E . No dichroism of the doublet observed by the light polarized in the [100] or [010] direction suggests rotation of the [ F - H ] axis during relaxation from the electronic b 1 u state, but not necessarily from the hole excited π g state.

By using a time resolving spectroscopic technique in picosecond range with a ruby laser, absorption spectra of NaI and NaBr single crystals have been observed to explore formation of [F-H] center pairs just after the excitation of crystals. Although the formation of self-trapped excitons at the lowest triplet state (STE L ) was observed, no F band due to [F-H] center pairs was detected in both crystals at 110 ps after the excitation at 80 and 373 K. We observed no F band in NaI 3 min after the excitation even at 373 K, while trace of the F band in NaBr at 296 K. Formation mechanism of [F-H] center pairs is discussed in connection with the configuration of the STE, i.e. with the “on-center” type STE L in these crystals.

The reverse photoreaction from batho-intermediate of bacteriorhodopsin has been investigated by picosecond laser flash photolysis at 6.5 K. Upon exciting batho-intermediate with 694 nm light pulse of 31 ±4 ps width, the increase of absorbance around 570 nm and concomitant decrease of that around 630 nm are observed without delay time. Based on the above result, the models for the primary photochemical reaction of bacteriorhodopsin are discussed.

Optical darkening and thermal bleaching of very thin a-As 2 S 3 films with 80±10 nm thickness have been investigated. A new absorption band with a half height width of 1.37 eV peaking at 3.32 eV occurs by illumination on the films with ∼3 eV or ∼4 eV photons at 80 K. The thermal behavior of this band is almost identical with that of the shift of the absorption edge in thick films reported by others. Another new band with a half height width of 1.95 eV peaking at 3.63 eV is produced by thermal annealing of the film showing the 3.32 eV band up to about 440 K. Possible optical and thermal kinetics to understand the results are discussed.

The growth times (τ g σ ) of the σ emission intensity in RbBr and KI crystals have been obtained to be 10±10 ps and 5±5 ps, respectively, under irradiation of a pulsed electron beam with the pulse duration of about 18 ps near liquid helium temperature. τ g σ includes both the life time of the conduction electron and the relaxation time of electrons from the conduction band down to the 1 Σ + u (σ u , 2 s σ g ) state of the self trapped exciton. τ g σ is discussed in connection with the formation mechanism of the F - H center pair. The decay times (τ d σ ) of the emission intensity in these crystals are 3.3±0.1 ns and 1.6±0.1 ns, respectively, in good agreement with those observed by using a pulsed proton beam with nsec pulse duration by Blair et al .

Formation times of F centers (τ F ) and self-trapped excitons (τ S T E ) in RbBr crystals have been measured under the excitation by two photon absorption of second harmonics from a mode-locked ruby laser. Such excitation produces the 2 p hot exciton directly in the crystal. τ F is 27 ±8 ps at RT and 30 ±8 ps at LHeT. τ S T E is not explicitly observed, but appears to be around 30 ps. Absorption spectra measured at about 100 ps after the excitation coincide to that of the conventional F band at RT, and consist mainly of the F band and of a small contribution of the self-trapped excitons at LHeT. τ F at RT and τ S T E at LHeT in RbI are both 30 ±10 ps. Possibly τ F in KBr at RT was 20 ±10 ps, but the absorption decayed with a time constant of 800 ±200 ps. The relaxation process of the 2 p hot exciton in RbBr is discussed in connection with the formation of self-trapped excitons and F centers.

By using a mode locked ruby laser, it has been shown that the self trapped excitons in KI crystals are created with a time constant of 210±15 psec at liquid helium temperature, and F centers are created with a time constant of 11±9 psec at room temperature, respectively. An additional absorption band growing with a time constant of 75±25 psec at liquid helium temperature and of 10±10 psec at room temperature was observed around 694 nm. This absorption band is bleached with a decay time of about 10 psec at both temperatures, but its origin was not clear. Formation processes of the self trapped excitons and of the F centers are discussed.

The decay time ( τ σ d ) of the σ emission intensity has been observed in temperature range of 5 K ∼200 K by using a pulsed electron beam in picosecond range. τ σ d in KI, NaCl, KBr and RbBr are 1.9 ns (7.4 K), 3.0 ns (∼5 K after Blair et al. ), 3.7 ns (5.7 K) and 4.0 ns (5.6 K), respectively, and start to decrease to 100 ps or less above nearly 70 K, 40 K, 30 K and 20 K depending on the crystal. The activation energy for such thermal quenching of τ σ d are 46.2 meV, 32.0 meV, 22.1 meV and 14.7 meV. The capture process of the conduction electron by the self-trapped hole and the non-radiative decay of the self-trapped exciton are discussed.

The relaxation process to the self-trapped excitons (STE) at the lowest triplet state (STE L ) and the formation process of the [ F + H ] center pairs have been investigated by the picosecond laser photolysis and nanosecond electron-beam radiolysis between 80 K and 296 K. Below ∼95 K, main products are STE L . Then, the formation time of STE L is 20±10 ps. Between ∼95 K and ∼140 K, STE at STE L are mainly produced, and F centers are slightly produced simultaneously. F centers grow also with a slow formation time corresponding to the lifetime τ T d of STE L . The typical slow formation time is 95±10 ns at 128 K. Above ∼140 K, STE L and F centers are still produced with formation time of 20±10 ps. However, the initial yield for STE L is reduced, while that for F centers increase, with increasing the temperature. Possible branching pathways from higher excited states (STE L ) into STE L and the [ F + H ] center pair are discussed.

Time resolved absorption spectra under the two photon excitation by the second harmonics from a mode locked ruby laser (∼40 ps) have been measured in KI. Instantaneous growth of the tail of the V k band within the pulse duration was observed with almost equal efficiency at LHeT and RT. The V k band turned into the Trip . band of the STE within about 500ps at LHeT, while into the F band within a few decades of ps at RT. These results suggest the V k center as the primary product, and no activation energy barrier for the holes to self-trap. The results are discussed by Toyozawa's model qualitatively.

It has been observed that the quantum efficiency of the F center emission in KI near liquid helium temperature drops drastically at the high energy region beyond the F absorption band. This drop suggests that, when the F center is excited to a higher vibrational level than the crossing point of the excited state and the ground state, it has a large probability to make dynamical nonradiative transition to the ground state above the crossing point.