Hiroshi Kashiwagi

Abinitio SCF CI calculations on the ground and π–π* excited states of the pyrrole molecule and its positive ion

Ab initio SCF π‐electron CI calculations are carried out on the ground and lower π–π* excited states of pyrrole and its positive ion. The four singlet states found at 5.9, 6.8, 7.2, and 7.4 eV by experiment are tentatively assigned as the 1B2, 1A1, 1B2, and 1A1 states, respectively. The calculated excitation energies for these four states are 6.0, 6.5, 7.1, and 7.3 eV, respectively. The first excited 1A1 state is a valence type state, whereas the other three states are Rydberg type states. The lowest excited state found at 4.35 eV by experiment is assigned as the 3B2 state, whose calculated excitation energy is 4.27 eV. The three lowest excited states of the positive ion due to π‐electron ionization are also investigated. The calculated ionization potentials are 8.10 (2A2), 8.94 (2B1), and 13.33 eV (2B1). They are in good agreement with experimental values of 8.21, 9.20, and 13.0 eV. The lowest two states are represented by la2→∞ and 2b1 →∞, respectively, whereas the second 2B1 state is a mixture of three...

Photodegradation mechanisms in laser dyes: A laser irradiated ESR study

The photodegradation mechanisms in xanthene dye-laser solutions are investigated at 77 K by the direct detection of a decrease in the concentration of dye molecules in the lowest excited triplet state (T 1 state) and of free radicals produced after degradation using an ESR technique under UV (3511 + 3638 A) or 5145-A laser irradiations. It is shown that the ESR spectral shape of the radicals induced by an UV laser beam is quite different from that of those induced by a 5145-A beam. For those radicals, the irradiation time and power dependences, the effects of dye and solvent molecular structures and the role of the T 1 state by the addition of cyclooctatetraene (COT) quenchers are examined in some detail. As a result the two following laser-induced photochemical reactions are proposed: in UV laser- or flashlamp-pumped dye lasers, the C-H bond rupture of a solvent molecule due to the energy transfer of a dye molecule in a higher excited triplet state, which is produced by a T-T absorption of a dye molecule in the T 1 state, results in a radical and a leuco compound of the dye. On the other hand, in the case of the 5145-A laser excitation, a partially reversible change of a dye molecule in the T 1 state produced by only a one-photon absorption results in another radical which depends on the chromophoric structure of the dye.

Amplification characteristics of an efficient discharge-pumped KrF laser

The significant increase of output energy of a UV-preionized KrF laser has been obtained by a change of electrode separation. This fact is explained by the measured impedance and deposit energy in a discharge. The efficient amplification has been performed by using a exactly synchronized amplification system. The maximum output energy was 0.75 J/pulse with a pulsewidth of 40 ns and a total efficiency of 0.8 percent. The saturation intensity and small-signal gain coefficient for KrF laser have been measured. The saturation intensity was 1.7 MW/cm2and the small-signal gain coefficient reached 4.7 percent/cm at a charging voltage of 35 kV.

Intersystem crossing rates and saturation parameters in the triplet state for rhodamine, fluorescein, and acridine dyes

The saturation of the population in the triplet state T1 using a laser excitation ESR technique was observed directly at 77 K. The saturation parameters Isat and the singlet‐to‐triplet intersystem crossing rate kST were determined for acridine red (AO), rhodamine S (RS), rhodamine B (RB), rhodamine 6G (R6G), fluorescein disodium salt (FDS), 2′,7′‐dichlorofluorescein (DF), eosin Y (EY), eosin B (EB), acridine orange (AR), and acridine yellow (AY) in rigid ethanolic solutions. On the basis of the results, the intersystem crossing mechanisims of rhodamine (type R), fluorescein (type F), and acridine (type A) types of the ten dyes are discussed.

Laser pumped magnetophotoselection in triplet dyes

Some observations have been made on the Δm=±2 and Δm=±1 ESR spectra in rhodamine (type R), fluorescein (type F), and acridine (type A) dyes, important laser active media, during laser excitation at 77 K. The results have revealed that the spectra and the lifetimes of the dyes in the T1 states in ethanol are affected significantly by the chromophoric structure. The root‐mean‐square value of zero‐field splitting parameters (ZFSP), D*, and the triplet‐state lifetime, τT, increase in the order type R < type A < type F and type F < type R < type A, respectively. By the method of magnetophotoselection, the Δm=±1 absorption lines of acridine orange and fluorescein disodium salt dyes are assigned to three directions along the molecular axes. Furthermore, the ZFSP values and the polarization results show that the π‐electron distribution is most strongly polarized along the long axis of the chromophore in the case of type F dyes.

Analysis on the steady‐state amplification of ArF and KrF discharge lasers

The saturation behavior of the steady‐state amplification of discharge‐pumped ArF and KrF lasers have been analyzed by numerical calculation taking into account absorption, spontaneous decay, and pumping. This analysis predicts the noticeable pulse‐width broadening due to amplification in good agreement with experiments. The small‐signal‐gain g0 and absorption coefficients a were determined simultaneously by the best fits of experimental saturation curves to numerically calculated ones with the results of g0=3.5%/cm and a=0.2%/cm for the ArF laser and g0=5.0%/cm and a=0.4%/cm for the KrF laser under the investigated conditions.

The selective excitation of lithium isotopes by intracavity nonlinear absorption in a CW dye laser

Abstract It has been experimentally demonstrated that 7Li atoms can be selectively excited by intercavity absorption in a Z-shaped CW dye laser cavity. In addition, it was found that the spectrum of the laser began to split at a critical temperature the intracavity 6Li vapour. As the temperature of the 6Li vapour is raised above the critical temperature, the splitting-interval rapidly increases.

Investigation of photodegradation mechanism in laser dyes by detection of triplet-molecules and free radicals induced by laser light

concentration of Rhodamine B (with the optical arrangement used). Experiments were also carried out on several pairs of dye mixtures, including Dichlorofluorescein as donor and Rhodamine B and DODC as acceptors, and using the 488.0 nanometer output of an argon ion laser to excite the donor alone. The results showed that excitation transfer rates were related (i) to the overlap of absorption and emission spectra of the acceptor and donor dyes respectively and (ii) to the dye concentration. The efficiency of excitation transfer, as measured by the total fluorescent quantum yield for the mixture, over the quantum yield for the donor dye alone was found to be -90% for single step transfer. This was the case for several of the dyes tested, including those used for three color laser operation. The high efficiency of the energy transfer processes makes the use of dye mixtures attractive for a variety of applications including three color lasers for display purposes, and for increasing the efficiency of flashlamp pumped dye laser systems by utilizing more effectively the wide spectral output of the pump bands.