Quenching of 1B 2u(6 1) vibrational level of the benzene molecule at 300 K by several colliders

By means of a three-level kinetic model developed previously, the nascent fine-structure branching ratios of K 4 2PJ doublets following photodissociation of KI at 248 nm can be determined accurately in the presence of foreign gases. With the forward and reverse Ar-induced collisional fine-structure mixing cross sections of 15 and 30 Å2 reported by Lijnse, and our measurement of Ar quenching cross section 0.81±0.08 Å2, the nascent branching ratio of the K 4 2P3/2 component is determined to be 0.611±0.002. Analogously, with the N2-induced collisional mixing cross sections of 100 and 190 Å2 reported by Lijnse and Hornman, and our measurement of N2 quenching cross section 18±2 Å2, the branching ratio of K 4 2P3/2 is determined to be 0.608±0.002. The agreement between these values confirms reliability of the kinetic model. However, a lack of confirmity is found in the presence of H2 quencher, using the collisional mixing cross sections of 53 and 75 Å2 reported by McGills and Krause, and our quenching cross section measurement of 2.8±0.3 Å2. The discrepancy is caused by the unsuitable adoption of mixing cross sections. Using the average branching ratio 0.610 determined from the cases of Ar and N2, the H2-induced collisional mixing cross sections can be evaluated to be 51±4 and 90±7 Å2. The obtained quenching cross sections by collisions with foreign gases are consistent with those reported elsewhere except for the case of Ar. The discrepancy of our Ar measurement of 0.81±0.08 Å2 from a reported value <0.07 Å2 does not cause a significant difference in the branching ratio determination; it is because the magnitude of the mixing cross sections are more than ten times larger than the relevant quenching cross sections, such that the dependence on the collisional quenching becomes insignificant.

The impact of pulsed versus continuous wave (cw) laser excitation on the photophysical properties of single quantum dots (QDs) has been investigated in an experiment in which all macroscopic variables are identical except the nature of laser excitation. Pulsed excitation exaggerates the effects of photobleaching, results in a lower probability of long ON fluorescence blinking events, and leads to shorter fluorescence lifetimes with respect to cw excitation at the same wavelength and average intensity. Spectral wandering, biexciton quantum yields, and power law exponents that describe fluorescence blinking are largely insensitive to the nature of laser excitation. These results explicitly illustrate important similarities and differences in fluorescence dynamics between pulsed and cw excitation, enabling more meaningful comparisons between literature reports and aiding in the design of new experiments to mitigate possible influences of high photon flux on QDs.

Quenching cross sections of the B3Π +( Ou+) v′ = 14 level of I 2 by H 2, CO, and CH 4

Accurate modeling of the operation of diode-pumped alkali lasers is a critical step toward the design of high-powered devices. We present precision measurements for the Cs-CH4 62P3/2 → 62P1/2 mixing cross section and the 62P3/2,1/2 → 62S1/2 quenching cross section, which are important parameters in understanding the operation and, in particular, the heat generated in a cesium vapor laser. Measurements are carried out using ultrafast laser pulse excitation and observation of fluorescence due to collisional excitation transfer in time is done using the technique of time-correlated single-photon counting. Mixing rate measurements are acquired over methane pressures of 10 - 40 Torr, resulting in a Cs-CH4 62P3/2 → 62P1/2 mixing cross section of (1.40 ± 0.08) × 10-15 cm2, while quenching rate measurements are carried out over methane pressures of 500 - 4000 Torr, resulting in a 62P3/2,1/2 → 62S1/2 quenching cross section of (1.57 ± 0.03) × 10-18 cm2. These results suggest only a slight contribution to the heating of a cesium vapor laser is due to Cs 62P quenching, contrary to previous studies. We also discuss additional possible sources of energy transfer from upper excited states of Cs.

Sol–gel deposition of multiply doped thermographic phosphor coatings Al 2O 3:(Cr 3+, M 3+) (M = Dy, Tm) for wide range surface temperature measurement application

Sol–gel deposition of multiply doped thermographic phosphor coatings Al 2O 3:(Cr 3+, M 3+) (M = Dy, Tm) for wide range surface temperature measurement application

The free charge carrier lifetime is a highly sensitive parameter that can be used for analyzing the function of semiconductor devices and monitoring the quality of wafer materials. In this context, time-resolved photoluminescence (TRPL) is presented as a technique for directly determining the free charge carrier lifetime with pulsed diode laser excitation and time-correlated single photon counting, employing highly sensitive single-photon avalanche detectors (SPADs). The full range in time-scales of charge carrier dynamics can be addressed with the capability to resolve luminescence lifetimes from approximately 50 ps up to several hundred microseconds. This is achieved with an instrument response function (IRF) as short as 100 ps and the capability of adjustable repetition rates in the pulsed laser excitation that can be adapted to the luminescence lifetime of the material. The technique is capable of correlating spectral information concerning material specific band gap transitions and transmission edges with the respective luminescence lifetimes in a specific spectral channel. This is particularly valuable for the analysis of multi-component systems. Furthermore, the general instrumentation can be combined with a raster scanning based microscope setup, which can be configured to cover lateral resolutions down to sub-μm scale and scan ranges from 100 microns up to several centimeters [1].

The cross sections for quenching the lowestn2P states of the alkali atoms Li, Na, K., and Rb by the inert gases He, Ne, Ar, Kr, and Xe are presented for 5 eV≦Ec.m.≦ 100 eV. These cross sections are derived from the corresponding cross sections for collisional excitation by applying the principle of microreversibility. Upper estimates for the quenching cross sections at thermal energies are given; in all studied cases the quenching cross sections are <8·10−3A2. These new upper limits are in most cases much lower than those obtained from other methods previously.

A pulsed dye laser has been used to excite OH and OD radicals to the A 2Σ+ state and the quenching and vibrational relaxation cross sections have been measured for several different buffer gasses. It was found that the quenching cross sections were reasonably constant within a given vibrational level and tended to increase at higher vibrational levels. The vibrational relaxation cross sections decreased for higher rotational states and were generally smaller for collisions in which V′ changes by two than for collisions where it changes by only one. Finally, the implications of these measurements to the detection of OH in the atmosphere are discussed.

The dynamics of the IR emission induced by excitation of the acetylene molecule at the 3(2) Ka2, A1Au<--4(1) la1, X1Sigmag+ transition was investigated. Vibrationally resolved IR emission spectra were recorded at different delay times after the laser excitation pulse. The observed IR emission was assigned to transitions between vibrational levels of the acetylene molecule in the ground state. Values of the relaxation parameters of different vibrational levels of the ground state were obtained. The Ti-->Tj transition was detected by cavity ring-down spectroscopy in the 455 nm spectral range after excitation of the acetylene molecule at the same transition. Rotationally resolved spectra of the respective transition were obtained and analyzed at different delay times after the laser excitation pulse. The dynamics of the S1-->Tx-->T1-->S0 transitions was investigated, and the relaxation parameter values were estimated for the T1 state.

State-to-state cross sections are reported for vibrational-rotational transitions for HF in collisions with He, at collisional energies of 0.5 and 1.0 eV. These were computed within the infinite-order sudden (IOS) approximation using adiabatic, distorted-wave techniques. Values are tabulated for the vibrational-rotational deexcitation sequences (v, j) → (v–1, 0), with v=1, 2, 3, 4 and j=0 – 40. These quenching cross sections can be used in conjunction with IOS factorization formulas to compute VRT cross sections for final rotational states other than jf=0. In addition to IOS results, vibrational quenching cross sections were computed using the much more simple breathing-sphere technique. The breathing-sphere results compare favorably to the more accurate IOS results, particularly as to energy dependence. This suggests a simple method of utilizing known quenching cross sections to predict values for different vibrational levels and/or collisional energies.

The room temperature rate constants for quenching of the fluorescence of H2, HD, and D2 B1Σ+u by 4He have been measured as a function of the initially excited rotational and vibrational levels of the hydrogen molecule. The effective quenching cross sections increase with increasing vibrational energy from about 1 Å2 up to a maximum of about 6 Å2. The effective cross sections for D2 (B, v′ = 0) were independent of the rotational level excited for 0 &amp;lt; J′ ≤ 7, and the cross sections for (v′ = 0, J′ = 0) were about 80% of the values for (v′ = 0, J′ ≳ 0) for all three isotopes studied. Quenching occurs via formation of an electronically excited (H2He)* collision complex followed by crossing to the repulsive H2(X)–He potential energy surface. The vibrational state dependence of the quenching cross sections fits a vibrationally adiabatic model for complex formation. From the vibrational state dependence of the quenching cross section, the barrier height for the quenching reaction is found to be 250±40 cm−1, and the difference in the H–H stretching frequencies between H2(B) and the H2–He complex at the barrier to reaction is 140±80 cm−1. Both values are substantially smaller than results from ab initio calculations. The rotational state dependence of the quenching cross sections suggests that quenching occurs with H2 rotating in a plane perpendicular to the relative velocity vector, in qualitative agreement with the rotational anisotropy of the H2(B)–He ab initio electronic potential energy surface.

Vibrational and electronic excitation of nitric oxide is studied experimentally using optical pumping by a CO laser. A mixture of nitric oxide and argon diluent is vibrationally excited in an optical absorption cell, by resonance absorption of CO laser radiation operating on a single line, in near resonance with one of NO(v=0 → v=1) fundamental band absorption transitions. Higher NO vibrational levels, not directly accessible to laser excitation, are populated by collisional vibration-vibration (V-V) energy exchange processes. Steady-state vibrational level populations and translational-rotational temperature in the cell are measured by Fourier transform infrared emission spectroscopy. At steady state, vibrational levels up to v~10 are populated. Steady-state vibrational temperatures up to Tv=3000-4500 K are maintained at low translational-rotational temperatures of T=330-440 K. At these conditions, UV emission (NO β and γ bands) is detected from the optically pumped cell, both at steady state and during pulsed laser excitation using a mechanical chopper to interrupt the laser beam. UV emission delay time is measured during pulsed excitation, relative to the laser pulse rise time. The results provide insight into kinetics of vibration-to-electronic (V-E) energy transfer in nitric oxide. Similar optical pumping technique can be used to study kinetics of dissociation of diatomic molecules at the conditions of extreme vibrational and electronic disequilibrium.

We report cross sections for fluorescence quenching and branching ratios for chemical reaction in collisional deactivation of Ga(5s1 2S1/2) by added gases at room temperature following resonance 4p1 2P○1/2 →5s1 2S1/2 excitation at 403.30 nm with a pulsed laser. Total quenching cross sections are obtained from Stern–Volmer analysis of fluorescence lifetimes. Chemical contributions to fluorescence quenching are investigated by a pump and probe technique involving saturation of the resonance transition by a pump laser pulse to produce a known fraction of gallium atoms in the excited state, and measurement of the depletion of the gallium atom concentration due to chemical reaction in the excited state. Relative concentration measurements in the presence and absence of the saturating pump pulse are by resonance fluorescence excitation by a probe laser pulse suitably delayed relative to the pump pulse. An analysis of the experiment in terms of rate equations shows how branching ratios for chemical reaction may be obtained from the depletion measurements. Fluorescence quenching cross sections (Å2) are large for N2O (99±10), CO2 (100±20), CH4 (55±6), C2H6 (77±20), C3H8 (100±20), n-C4H10 (130±20), and C2H4 (75±20), moderately large for CO (11±4) and N2 (8±2), and very small for CF4 (&amp;lt;0.3) and H2 (&amp;lt;0.05). Among the efficient quenchers only C2H4 showed no detectable contribution from chemical quenching. Branching ratios for chemical removal of Ga(5 2S1/2) are N2O (0.96+0.04−0.1), CO2 (0.55±0.1), CH4 (0.27±0.07), C2H6 (0.33±0.1), C3H8 (0.26±0.1), and C2H4 (0.0+0.05−0.0). Results for H2 and the alkane hydrocarbons are discussed with reference to simple concepts of orbital interactions in the entrance channel.

Mercury atoms in the 63P0 state were produced from the Hg(63P1) state by collisions of the second kind with N2 under steady-state illumination with 2537 Å light in a static system. C2H4 was found to quench Hg(63P0) atoms, with a rate constant of 2.4 × 1013 cc mol−1·sec−1, forming excited C2H4 which decomposes to C2H2 and H2 with a mechanism similar to the Hg(63P1) photosensitized decomposition. The reduction in the yield of this reaction due to the addition of various gases was used to determine cross sections for the deactivation of Hg(63P0) atoms by H2, D2, CH4, C2H6, C3H8 CH3CD2CH3, c-C3H6, C(CH3)4, n-C4H10, and i-C4H10. The quenching cross sections relative to C2H4 are, respectively, 4.94 × 10−2, 4.72 × 10−2, &amp;lt; 1.0 × 10−5, 4.0 × 10−4, 3.9 × 10−3, 3.1 × 10−4, 1.8 × 10−4, 4.3 × 10−4, 6.8 × 10−3, and 2.2 × 10−2. Relative quenching cross sections for some of the preceding were obtained also from measurement of Hg(61P0) induced electron emission from Ag, and the results were in agreement with the first method. In general, Hg(63P0) reactions are similar in nature to other excited metal atom–molecule reactions. However, they exhibit much lower quenching cross sections and a more marked dependence on the strength of the C–H bond than do corresponding Hg(63P1) reactions. The inadequacy of simple models for quenching based solely upon conservation of total electronic angular momentum is discussed. The importance of both total excitation energy and C–H bond energy is shown by a comparison of the quenching cross section of Hg(63P0), Hg(63P1), andHg(61P1) by the alkanes.