Collisionless Conditions Research Articles

Kinetic energy disposal in the unimolecular IRMPD of methyl nitrite in a pulsed molecular beam

Methyl nitrite, CH 3 ONO, was dissociated by infrared laser pulses of well defined intensity under collision-less conditions in a pulsed molecular beam. Doppler-resolved laser-excited fluorescence spectroscopy determined the kinetic energy of the nitric oxide fragments formed in particular quantum states. The observed Doppler profiles were Gaussian over two e-foldings and, when converted to translational temperatures, corresponded to temperatures in the range of 260 to 350 K for states with 40 to 1308 cm −1 of rotational energy; no significant correlation was observed between rotational, spin-orbit or lambda doublet state and kinetic energy.

Ultrafast molecular relaxation of isolated stilbene: measurements by picosecond pump-probe techniques

In this Letter, we report measurements of the picosecond relaxation time of trans-stilbene in S1 under collisionless conditions. By acurate determination of the zero-time in our picosecond pump-probe experiments we obtained r = 3 ± 0.5 ps, which is entirely consistent with our earlier measurements of IVR rates of stilbene in bulbs and beams. This establishes the molecular nature of the decay in these picosecond pump-probe multiphoton ionization (or mass spectrometry) experiments.

Laser and ion beam studies of liquid sodium surface chemistry and Na 2 evaporation

Laser spectroscopy was used under collisionless conditions to study the internal energy distributions and macroscopic evaporation rate for Na2 evaporating from liquid Na at 480 K under clean and reactive conditions. The Na2 rotational and vibrational population distribution always could be characterized by the surface temperature. The Na2 macroscopic evaporation rate, R, was unaffected by exposure to O2 at pressures below ∼ 10−6 Torr; R increased at 10−6 to 10−5 Torr O2. Benzene vapor decreased R significantly at low exposures. Secondary ion mass spectrometry of the liquid Na surface using 3 keV Ar+ was also examined. Only upper bounds on the very low secondary ion yields for clean Na and Na + O2 could be determined. The ion yields for Na + C6H6 were appreciable under 10−6 Torr C6H6 only after reducing the temperature to ∼ 390 K. The surface chemistry and mechanisms responsible for the variation in macroscopic evaporation rates and ion yields are discussed.

Vibrational Level Populations of Hydrogen Fluoride Produced in Infrared Multiple-photon Decompositon of C2H5F

Abstract CO2 TEA laser-induced photolysis of C2H5F was studied by using a time-resolved infrared fluorescence technique. Molecular elimination of vibrationally excited hydrogen fluoride (HF*) from C2H5F* occurs in the process. Rotational temperatures and vibrational level populations of HF* are estimated from observed fluorescence spectra by computational fitting. Higher vibrational levels are populated to a less extent in the decomposition under a collisionless condition. When the pressure of C2H5F is increased, however, population inversion is allowed among some levels. Possible reaction mechanism is discussed for the production of HF* at extraordinarily high vibrational levels.

Molecular Beam Opto‐thermalStudy of the Multiple‐photonInfrared Excitation of CF3Br

The multiple‐photon excitation (MPE) of CF3Br has been studied in strictly collisionless conditions by means of the molecular beam optothermal technique. The MPE spectrum shows strong structures which have been attributed to different orders of multiple‐photon resonances. The experimental results have been compared with an available theoretical calculation based on the “heat‐bath feed‐back” model. The effect of the rotational population and of the vibrationally excited “hot bands” on the MPE yield is discussed.

Recoil fluorine-18 chemistry. 14. High-pressure investigation of perfluoroethane (C2F6)

With gas-phase C/sub 2/F/sup 6/ nuclear recoil /sup 18/F undergoes F-for-F, F-for-CF/sub 3/, and F-for-2F, hot atom elementary reactions with respective primary yields of 13.8 +/- 0.2%, 0.68 +/- 0.03%, and 1.90 +/- 0.05%. Under collisionless conditions, 91 +/- 2%, 57 +/- 2%, and 35 +/- 3% of the nascent products from these relaxations decompose. At pressures below 1 ktorr, 22 +/- 2% of the F-for-F product, CF/sub 3/CF/sub 2//sup 18/,F undergoes complete atomization through sequential C-C and C-F bond dissociations. Above 50 ktorr, caging reactions enhance the yields of most of the observed products. 49 references, 8 figures, 6 tables.

IR Multiphoton-induced retro Diels—Alder dissociation of decafluorocyclohexene by a pulsed CO 2 laser

The retro Diels—Alder dissociation of decafluorocyclohexene into tetrafluoroethylene and hexafluorobutadiene was experimentally investigated. The nascent hexafluorobutadiene formed after dissociation isomerizes to thermodynamically stable hexafluorocyclobutene. The wavelength-dependent and energy-dependent dissociations were studied. Experiments over a range of pressure showed that collisions have a detrimental effect on the rate of dissociation. Under collisionless conditions, the threshold fluence for the decomposition of decafluorocyclohexene at the R(20) line of the 10.6 μm CO 2 laser line was found to be 0.92 J cm −2.

Strong dependence of ir multiphoton absorption and dissociation yield of SF6 molecule on intensity (duration) of exciting laser pulse

The role of played by the intensity of the exciting TEA CO2 laser pulses in the processes of ir multiphoton absorption (MPA) of SF6 molecules cooled toT R≃40 K andT v≃160 K in a pulsed supersonic jet and the dissociation of SF6 in a bulk atT≃300 K under essentially collisionless conditions have been investigated. A strong dependence of MPA and the dissociation yield on pulse intensity were observed. The frequency dependences of the intensity effects were studied.

A study of the dynamics of UV laser photolysis of NOCl and NOBr

The photodissociation of NOCl and NOBr using lasers operating at 193 (ArF), 249 (KrF), 308 (XeCl), and 337 nm(N2) has been studied under collisionless conditions by measuring the infrared fluorescence of the NO product. Little difference between the results for NOBr and NOCl was observed and all results indicate a linear dependence of photodissociation yield on laser power. The extent of NO vibrational excitation observed was quite large in photodissociation utilizing the ArF and KrF laser lines, with states up to v≥16 observed with ArF and up to v≥14 with KrF. The XeCl and N2 lines produced much less NO excitation, up to v≥5. Statistical, half-collision, and Franck–Condon models of the photodissociation dynamics were studied, with best agreement with experiment for dissociation at 193 and 249 nm being obtained using a Franck–Condon model. An excited potential function for NOCl in which the NO properties were taken to be close to that of NO(4Π) was found to predict the shape of the dissociated NO vibrational distribution accurately at both 193 and 249 nm. Photolysis at 308 and 337 nm could also be explained by a Franck–Condon model using the NO(A2Π) state to model the NOCl excited state.

ChemInform Abstract: INFRARED MULTIPHOTON PHOTOPHYSICS: DECOMPOSITION OF CNF2N+1I (N = 1, 2, 3)

The title compounds have been subjected to unfocused radiation from a high‐power CO2‐TEA laser under collisionless conditions in a mass spectrometric flow system (VLPO/ method). We report absolute photodissociation yields as a function of fluence for CF3I and C3F7I. In the case of CF3I, we find high yields (100%) at fluences ?4 J/cm2, whereas for C3F7I yields level off at 20%–30%, depending on the wavelength. Secondary photolysis of Ċ2F5 and Ċ3F7 at fluences ?2.5 J/cm2 yields CF2 and ĊF3, but no C2F4. Interpretation of data in terms of a log–normal cumulative distribution function of reaction times or fluence suggests that CF3I interacts homogeneously with the laser field at 1070 and 1083 cm−1, but possibly inhomogeneously at 1076 cm−1, whereas C3F7I represents an inhomogeneous interaction case at all wavelengths. Based upon our photodissociation yields and the validity of a master equation representation, we calculate the infrared absorption spectrum of C3F7I as a function of internal energy.

Photodissociation of formaldehyde in a molecular beam

Formaldehyde photopredissociation was studied under collisionless conditions by the technique of crossed laser and molecular beams. Detection of the molecular product CO after excitation of H2CO near the S1 origin gives strong support to the sequential coupling model for fast nonradiative decay of S1 states through broadened S0 levels to the continuum. For H2CO excitation at 283.9 nm, formation of the radical product HCO dominates dissociation to molecular products by at least one order of magnitude.

Infrared multiphoton photophysics: Decomposition of CnF2n+1I (n = 1, 2, 3)

The title compounds have been subjected to unfocused radiation from a high-power CO2-TEA laser under collisionless conditions in a mass spectrometric flow system (VLPO/ method). We report absolute photodissociation yields as a function of fluence for CF3I and C3F7I. In the case of CF3I, we find high yields (100%) at fluences ?4 J/cm2, whereas for C3F7I yields level off at 20%–30%, depending on the wavelength. Secondary photolysis of Ċ2F5 and Ċ3F7 at fluences ?2.5 J/cm2 yields CF2 and ĊF3, but no C2F4. Interpretation of data in terms of a log–normal cumulative distribution function of reaction times or fluence suggests that CF3I interacts homogeneously with the laser field at 1070 and 1083 cm−1, but possibly inhomogeneously at 1076 cm−1, whereas C3F7I represents an inhomogeneous interaction case at all wavelengths. Based upon our photodissociation yields and the validity of a master equation representation, we calculate the infrared absorption spectrum of C3F7I as a function of internal energy.

Influence of collisions on coherent IR multiphoton absorption in thiophosgene

Absolute changes due to IR pumping in the intergrated rotational population of the vibrationless level (000000) of thiophosgene have been measured under beam conditions and bulb conditions (1–1000 mTorr). Under collisionless conditions, large decreases are observed which reflect depopulation of all rotational states at a single CO2 laser frequency. At higher thiophosgene pressure, less depopulation occurs; zero depopulation of the vibrationless level is observed at pressure ? 300 mTorr within experimental uncertainty (? 3%). The latter agrees with measurements of the average number of photons absorbed from photoacoustic data (0.013±0.001) obtained at a pressure of 500 mTorr. The results support a mechanism of coherent pumping in the low density of states regime and the existence of a long range, collision-induced dephasing interaction which has a cross section ≳≳ gas kinetic.

Surface oscillations of a fermi liquid

Surface oscillations of a normal Fermi liquid are considered within the framework of the Landau phenomenological theory of a Fermi liquid. A procedure for describing surface phenomena is formulated which is based on the self-consistent field kinetic equation involving successive separation of the surface parts of various characteristics of the liquid. Boundary conditions at the free surface are derived. An expression for the surface tension coefficient is obtained. Oscillations of a Fermi liquid under hydrodynamical and collisionless conditions are investigated and the density and current distributions in the presence of such oscillations are found. The role of system finiteness effects is discussed. It is concluded that weakly damped low-lying surface vibrations in nuclei exist only as a result of nuclear finiteness. The contribution of surface fluctuations to the free energy of a Fermi liquid is calculated. The analytic dependence of the surface tension coefficient upon the temperature is predicted in the low-temperature limit.

The origin of fluctuations and cross-field transport in idealized magnetic confinement systems

The study of plasma fluctuations and confinement in idealized systems such as octupoles and levitrons has contributed to the understanding of cross-field transport processes. The linear theory of plasma instabilities that cause fluctuations is well developed and can predict growth rates γ and wavelengths θ x around lines of force. However, the theoretical prediction of cross-field transport coefficient D ± is restricted to quasilinear estimates of upper bounds (for example, D = 1 2 γ λ x 2 ) because of the complexity of the full nonlinear calculation. Such quasilinear estimates usually far exceed the measured values and are of limited worth. A general view of the results from octupole and levitron experiments shows that under collisional conditions ( λ ei / L < 0 ) the diffusion coefficient, D , scales in the same way as classical collisional diffusion ( D α n / T e 1 2 B 2 ). Agreement is closely approached in many cases, sometimes even in the presence of fluctuations. Under collisionless conditions ( D α n / T e 1 2 B 2 ), Bohm diffusion scaling ( D α T e / B ) is found in the few cases where the scaling law has been determined. This behaviour is consistent with the general scaling laws of Connor & Taylor (1977) but is not understood in detail. In addition there is evidence, both experimental and theoretical, that long-wavelength low-frequency electric fields (convection cells) can be generated nonlinearly from high-frequency fluctuations and can contribute to cross-field transport

Internal energy of hydroxyl radicals desorbing from polycrystalline Pt surfaces

The internal energy distribution of OH radicals desorbing from a polycrystalline Pt surface at 1130 K was measured by laser-induced fluorescence at pressures near collisionless conditions. Both the rotational and vibrational temperatures are equal to that of the Pt, suggesting that the rate of energy randomization of OH with the surface is greater than the rate of desorption.

Peak power dependence of rovibronic populations below the quasicontinuum during IR multiphoton absorption

Rovibronic levels of the ground state of thiophosgene, perturbed by IR multiphoton absorption, are observed under collisionless conditions by laser-induced fluorescence. Evidence is presented that at peak powers of the CO2 laser≳3 MW/cm2 an energy distribution is produced in which the vibrational populations of levels v=0–4,6,8 of the mode ν4 are depleted or not populated, even though the pumped mode is 2ν4. Only at lower peak powers (longer pulse duration) are population increases observed. Under the latter conditions, it appears that 3ν4 is populated to a smaller extent than [ν1+ν3] to which it is coupled by strong Coriolis interaction. At room temperature no evidence for rotational ’’hole burning’’ effects during IR pumping are observed in the vibrationless level. The data suggest, in fact, that all rotational levels interact with the laser field, even when thiophosgene is rotationally cooled.

Energy redistribution observed in infrared multiphoton excited C2F5Cl

The emission spectrum of infrared laser multiphoton C2F5Cl recorded under nearly collisionless conditions is shown. Emission from both unrelaxed discrete states and intramolecularly relaxed quasicontinuum levels are identified by comparison with thermally prepared C2F5Cl. The discrete states appear composed principally of ν4=1 levels indicating that rapid intramolecular energy redistribution begins at ν4=2,3 and state densities of ∼400/cm−1.

Influence of collisions and pulse intensity on multiple photon absorption in SF 6

Using a pyroelectric detector, the multiple photon absorption (MPA) of the SF 6 molecule in a wide range of pressures (10 -3 -1 torr) has been studied. The significant role of collisions in MPA has been shown. The fraction of molecules excited under essentially collisionless conditions has been defined. It is shown that under collisionless excitation of SF 6 ( p < 10 -2 torr) at energy fluences E < 10 -1 J/cm 2 the intensity of the laser pulse plays the essential role, while in presence of collisions MPA is determined mainly by the energy fluence in the pulse.

Isotope, electric field, and vibrational state dependence of single rotational level lifetimes of S1 formaldehyde

Additional single rovibronic level lifetimes of S1 H2CO and D2CO have been measured under collisionless conditions. The H2CO 41 lifetimes vary at least a factor of 150, from 20 nsec to 3.10 μsec. The observed D2CO 41 lifetimes fluctuate about ±20% around a mean value of 6.2 μsec, which is probably close to the pure radiative lifetime. In contrast, the observed D2CO 43 lifetimes vary from 1.09 to 2.46 μsec and the 2143 lifetimes vary from 212 nsec to 1.61 μsec. The onset of rotational state lifetime fluctuations in D2CO thus coincides with the high pressure D2+CO photochemical threshold. All of these results are explained in terms of a collisionless sequential decay mechanism, S1→S0→H2(D2)+CO. The last step probably involves tunneling through a barrier for the lower energies studied. For several H2CO 41 rotational levels application of a uniform external electric field of 0–4.6 kV/cm can change the fluorescence lifetime by at least a factor of 4. This result is understood in terms of small (≲0.05 cm−1) shifts in S1−S0 energy spacings. Quantitative estimates of S1−S0 intramolecular couplings, S0 widths due to dissociation, and S0 level spacings are derived in favorable cases.