Optic-acoustic Effect Research Articles

Vibrational relaxation of N 2O by the spectrophone method

The optic-acoustic effect was used for measuring vibrational relaxation times of N 2O, both pure and in collisions with atoms. By exciting the (10°0) asymmetric stretching mode and assuming a rapid V- V equilibrium with the bending (01°0) mode, one obtained vibration-translation relaxation times, reduced to one atmosphere, of τ VT = 0.87, 0.14, 1.38, 6.8 and 17.7 μs. in collisions of N 2O with N 2O, He, Ne, Ar and Kr, respectively. The corresponding experimental transition probabilities are compared with theoretical predictions. Through excitation of the (00°1) mode it seems possible to conclude that the de-excitation of this energy level takes place via the (03 l 0) and/or the (11 10) levels. The obtained vibration-vibration relaxation times are τ VV = 1.84, 3.8, 6.8, 12.6 and 23 μs for, respectively, pure N 2O and mixtures with He, Ne, Ar and Kr, at one atmosphere.

Photophysical processes in the vapour phase measured by the optic–acoustic effect. Part 4.—Radiationless processes from the lowest singlet and triplet states of some simple benzene derivatives

The lifetimes in the vapour phase of the vibrationally equilibrated lowest triplet states of toluene and ethylbenzene are reported. Models for the pressure-independent and pressure-dependent parts of the triplet state deactivation are briefly discussed. For [1H6]benzene, [2H6]benzene and toluene the nature of the radiationless processes following intersystem crossing from the first excited singlet state S1 are analysed.The quantum yields of intersystem crossing from S1 into the triplet manifold are measured (248 nm excitation) as 0.85 ([1H6]benzene), 0.74 ([2H6]benzene) and 0.77 (toluene) at high pressure.

Photophysical processes in the vapour phase measured by the optic–acoustic effect. Part 7.—Vibrational–translational relaxation in I2, X state, and collisionally induced predissociation in the B state

The spectrophone signal in gaseous I2, at 0.3 Torr and 295 K, and following modulated excitation at 546 nm, is analysed in terms of contributions from thermal energy release and from particle density increase due to predissociation. The former contribution is shown to be dominant and allows the measurement of the vibrational–translational relaxational relaxation time in I2(X state). The measurement is novel in that the excess translational energy produced by predissociation is frequency-monitored as it equilibrates with the thermally accessible vibrational levels. The extent of energy released is analysed to show that in the collisional predissociation process almost no energy (< 5%) is transferred to the vibration of the I2 collision partner. There is no evidence for a significant yield of direct (Xâ†�B, I2) intersystem crossing. “Hot” I atom excitation of I2 vibrations is also discussed.

Photophysical processes in the vapour phase measured by the optic–acoustic effect. Part 6.—Analysis for photofragmentation and photocombination with examination of mass and thermal diffusion to the cell walls

Following gas-phase photofragmentation in the spectrophone the resultant acoustic signal is due to the increase in particle density as well as the concomitant release of translational energy. The relative magnitudes of these two contributions are examined theoretically and the effect of mass diffusion to the walls on the amplitude and phase of the acoustic signal is considered. Similar equations for the decrease in particle density due to photocombination are briefly discussed. By analogy with the mass diffusion analysis, thermal diffusion to the walls is examined and calculations on the amplitude and phase effects of such thermal loss are given.

Vibrational relaxation in CD 4 and CD 4-rare gas mixtures

Using the optic-acoustic effect, measurements of the relaxation time were made after exciting the v 4 mode of CD 4. This was done for pure CD 4 and for mixtures of CD 4 with He, Ne, Ar and Kr, the results being 3.9±0.2, 1.8±0.2, 18±2, 70±15 and ≳ 100 μs at 1 atm, respectively. From the relaxation time dependence on mass as well as from a comparison between results for CH 4 and CD 4 it is concluded that energy transfer takes place via a vibration-rotation-translation (V-R-T) mechanism. Experiments were also conducted to study the most probable deexcitation path from the higher energy mode, v 3, to translation. This was done for CD 4-CD 4 as well as for CD 4-atom collisions. For pure CD 4 deexcitation occurs via the lower energy group ( v 2, v 4), while for CD 4 mixtures it may also occur via the higher energy group ( v 1, 2 v 2, v 2 + v 4, 2 v 4).

Vibrational relaxation of CO 2 in CO 2-N 2 and CO 2-O 2 mixtures

Vibrational relaxation in CO 2-N 2 and CO 2-O 2 mixtures is studied via the optic-acoustic effect. The relaxation of the bending vibration of CO 2 by N 2 and O 2 can be explained by a simple one-step relaxation. For CO 2-N 2 a relaxation time of 12.8 ± 1.5 μs atm is obtained, for CO 2-O 2 8.8 ± 0.3 μs atm. The relaxation of the asymmetric stretching vibration of CO 2 by N 2 can be described by a two-step mechanism. The result of 11.9 ± 0.25 μs atm for the relaxation time is in reasonable agreement with laser-fluorescence results. The frequency and pressure dependences of the relaxation observed in the CO 2-O 2 mixtures cannot be explained by a two-step mechanism.

Photophysical processes in the vapour-phase measured by the optic-acoustic effect. Part 2.—Triplet state yield and lifetime in high pressure biacetyl vapour

The photophysical processes in biacetyl vapour are studied using the optic-acoustic effect following excitation with radiation of wavelength 436 nm. An analysis is developed to apply the technique to second-order reactions, and the rate of triplet–triplet annihilation measured as 2.3(±0.3)× 1011 dm3 mol–1 s–1 at 298 K. A triplet yield of 0.97 ± 0.015 is determined.

Photophysical processes in the vapour-phase measured by the optic-acoustic effect. Part 1.—The model and apparatus for the study of radiationless processes

The use of the optic-acoustic effect in the study of radiationless processes from excited electronic states is discussed. A multi-state relaxation model is developed, and it is shown in some detail for a system of simple photophysical processes which are typical of those observed for aromatic molecules. The apparatus, involving the use of a capacitance microphone, is described, and the response of the microphone, as a function of both modulation frequency and gas pressure, is theoretically and experimentally determined.

Photophysical processes in the vapour-phase measured by the optic-acoustic effect. Part 3.—Radiationless processes from the lowest singlet and triplet states of [1H6]benzene and [2H6]benzene

Optic-acoustic measurements on the triplet state relaxation of [1H6] benzene and [2H6] benzene are described. The respective triplet deactivation rates in s–1 are 1.7 × 103+ 6.6 × 107[C6H6] and 1.9 × 103+ 2.1 × 107[C6D6] with [C6H6][C6D6] in mol dm–3. The concentration dependent term is discussed in terms of triplet excimer formation which may facilitate the passage through an isomeric intermediate to ground state benzene. For excitation at 40400 cm–1 the triplet quantum yield tends to zero at zero pressure. An explanation of this is given, not involving reversible radiationless processes, but in terms of both pressure-dependent and pressure-independent internal conversion rates out of the vibrationally excited singlet state. With this model internal conversion rates in s–1 are 1.7 × 107+ 1.5 × 1010[C6H6] and 1.3 × 107+ 1.3 × 1010[C6D6] for excitation at 40400 cm–1.

Relaxation of the asymmetric stretching vibration of CO 2 in pure CO 2 and in mixtures of CO 2 with noble gases

Using the optic—acoustic effect experiments were made in which the relaxation of the (0, 0, 1) level of CO 2 was studied in pure CO 2 and in mixtures of CO 2 with He, Ne, Ar, Kr and Xe. This was done via the amplitude—frequency response method. Improved equations describing the frequency dependence of the sound amplitude are derived. The relaxation times obtained for the de-excitation of the asymmetric stretching vibration of CO 2 by CO 2, He, Ne, Ar, Kr and Xe are: 4.0 ± 0.1, 15.0 ± 0.3, 27.5 ± 0.9, 43 ± 1.5, 82 ± 8 and 190 ± 50 μs at 1 atm, respectively. The frequency and pressure dependences of the amplitude give an indication of the processes by which the energy of the (0, 0, 1) mode of CO 2 is distributed. For the mixtures the energy is transferred to the (0, 2, 0) and (1, 0, 0) levels. For pure CO 2 the situation is more complex because of possible intermolecular energy transfer.

Triplet-state lifetime of vapour-phase benzene

A technique based on the optic-acoustic effect has been used to measure the triplet state lifetime of benzene over a range of pressure. The rate-constant for the deactivation of the triplet state by collisions with ground-state benzene is 1.1 × 10 −13 sec −1 cm 3 molecule −1, which corresponds to a collisional efficiency of 1.6 × 10 −4, and the pressure-independent radiationless rate-constant is approximately 1.7 × 10 3 sec −1. The collisional efficiency is discussed in terms of triplet excimer formation.

Kinetic theory of vibrational relaxation in a radiation field: The optic-acoustic effect

A microscopic derivation is presented of the rate equations governing vibrational relaxation occurring in the optic-acoustic effect. Detailed expressions applicable to the spectrophone experiment are given both for an excitation source consisting of a broadband radiation field and for laserdriven systems. It is clear from the present treatment that no real advantage accrues from the use of laser excitation sources in the standard spectrophone experiment, due to the resultant strong dependence of the driving force itself on the mechanical chopper frequency. For broadband radiation field the dependence on the chopper frequency is removed and the standard result containing the Einstein coefficient of induced absorption is recovered. The spectrophone response for the simplest case of a two-level system is given explicitly and its similarity to phenomenologically derived expressions is pointed out.

Relaxation of the bending vibration of CO 2 in pure CO 2 and in mixtures of CO 2 with noble gases

Applying the amplitude-frequency response method of the optic-acoustic effect, relaxation of the bending vibration of CO 2 has been studied in pure CO 2 and in CO 2-noble-gas mixtures. The relaxation time in pure CO 2 of 6.7 ± 0.2 μs atm is in good agreement with several other experimental determinations. The obtained relaxation times for CO 2-Ne, CO 2-Ar, CO 2-Kr and CO 2-Xe are 10.2 ± 0.3, 54 ± 2, 200 ± 10 and 190 ± 15 μs atm, respectively. The increase of the measured relaxation times as a function of the reduced mass is less steep than the one calculated via the S.S.H. theory.

Measurement of vibrational relaxation times in the spectrophone by the amplitude-frequency response method

The amplitude-frequency response method of the optic-acoustic effect has been used to study vibrational-vibrational and vibrational-translational relaxation times in CO 2 and mixtures of CO 2 with He, Ar and Kr. The results show that relaxation of the asymmetric stretching vibration of CO 2 towards translation occurs via a two-step relaxation process. Evidence has been found that the first step is the transition from the (0, 0, 1) state of CO 2 to the (0, 2, 0) or the (1, 0, 0) state. The second step is the degradation of the bending vibrational energy to translational energy.