Stephen E. Bialkowski

The determination of mass transport coefficients and vibrational relaxation rates of species formed in laser photolysis experiments

A simple analytical solution of the equations which govern the formation, collisional relaxation, and mass transport rates of species produced in radially symmetric laser‐induced processes is given. These equations are specifically applied to the CO2 laser‐induced dissociations of CF2HCl and C2F3Cl dilute in argon. The concentration of the vibrational ground state of the CF2 radical product was probed as a function of time and pressure both during and after the photolyzing laser pulse by the laser‐excited fluorescence technique. From these measurements, the vibrational relaxation rate of ? CF2 in argon was determined to be kVT=2.0×10−15 cm3 sec−1 and its diffusion coefficient was found to be D=90 cm2 Torr sec−1 in argon.

Energy partitioning in CO2 laser induced multiphoton dissociations: Energy of X̃/CF2 and X̃/CFCl from CF2CFCl

We have measured the vibrational (v), rotational (J,K), and translational energy, (ET), of the X CF2 and X CFCl fragments formed in the CO2 laser induced multiphoton dissociation of CF2CFCl (chlorotrifluoroethylene): CF2CFCl→CF2 (v,J,K)+CFCl(v,J,K)+ET(v,J,K), which was the only detectable reaction path for CF2CFCl. More vibrational energy (Ev) appears in CF2 than in CFCl. Direct spectroscopic measurements of populations in levels 0<ν2<7 show that Ev is distributed statistically in the bending mode (ν2) of CF2 according to P (Ev) =exp(−Ev/kTv), where P (Ev) is the probability of a CF2 product being formed with a particular amount of energy in ν2, and the vibrational temperature which characterizes the nascent distribution is Tv(ν2) =1860±250 °K. A vibrational relaxation method was used to accurately determine fo, the fraction of CF2 and CFCl molecules initially formed in the ground vibrational level. The measurements of fo showed that the energy in the stretching modes (ν1 and ν3) of CF2 is not character...

Collisionless formation and rovibronic relaxation of CH and OH from the ir multiphoton photolysis of CH3OH

A CO2 TEA laser has been used to initiate the collisionless multiphoton dissociation of CH3OH between 1000 and 10 mtorr. The appearance of OH(X 2Πi) 50±20 nsec after the laser pulse, independent of initial CH3OH pressure (50–200 mtorr), suggests the primary dissociative channel CH3OH+nhν→CH3 +OH(X 2Πi); although CH3 could not be correspondingly confirmed. The appearance of CH(X 2Πr) 70±20 nsec after the OH(X) appearance, independent of initial CH3OH pressure (70–400 mtorr), suggests secondary collisionless dissociation. Initial rovibronic distributions of OH(X) and CH(X) were determined, as well as characteristic decay time constants. The observation of the relaxation of OH(X) over a 10 μsec time interval after radical onset, allowed the separate characterization of collisional rotational relaxation and translational diffusion.

Thermo-Optical Spectrophotometries in Analytical Chemistry

I. INTRODUCTION A. Background The incorporation of lasers as light sources in analytical spectroscopic instrumentation has resulted in spectacular detection limits. These detection limits arise from several unique properties of laser radiation. As an example, the spectral purity and high power of the laser facilitates detection of individual gas phase atoms with isotopic resolution by both fluorescence and ionization techniques.1−3 The temporal coherence of the laser has been used to discriminate signals generated by very fast scatter processes from relatively slower luminescence relaxation.4, 5 In addition, the spatial coherence of the laser allows measurement by thermo-optical techniques of very small absorbance, the subject of this review. A recent review in this journal considered the related technique of photoacoustic spectroscopy, another high-sensitivity absorbance method.6

The gas phase laser induced fluorescence spectroscopy of chlorofluoromethylene

The CFCl radical has been produced in the gas phase by both IR multiphoton photolysis of C2F3Cl and He metastable reaction with C2F3Cl. Single vibronic level fluorescence and excitation spectra taken of this species have yielded sufficient information to determine certain vibronic constants. The excited state was found to have an origin of T0=25 283±5 cm−1 with vibrational frequencies: ω2′=394±3 cm−1 and ω3′=739±5 cm−1. In the ground state, the vibrational frequencies were found to be: ω3″=118±10 cm−1, ω2″=448±6 cm−1, and ω3″=750±6 cm−1. The rotational structure of the vibronic bands is similar to those of band transitions of a near prolate top, and the electronic transition has been tentatively identified as ? 1A″←? 1A′ by analogy to similar CF2 transitions. The radiative lifetime of the excited state, τr=700±10 nsec, was found to be independent of the rovibronic excitation frequency. The change in the major rotational constant was found to be A′−A″=1.8±0.2 cm−1.

The Infrared Multiphoton Photochemistry of Methanol

CO2 TEA laser has been used to initiate the multiphoton dissociation of CH3OH, both pure and with NO added as a free radical scavenger. The decomposition of CH3OH at the high power density of the focused laser radiation results in molecular and free radical initiated products. Decomposition through molecular intermediates appears to proceed via CH3OH+nhν→CH2O*+H2 and CH2O* →CO+H2, and comprises ∼90% of the consumed CH3OH in ∼3000 laser pulses. The radical initiated process CH3OH+nhν→CH3 +OH ultimately results in the stable products C2H4, C2H2, and CH4 to an extent of ∼10% of the CH3OH consumed. Luminescence from the focal zone is due to emission from OH†, CH†, C†2, and possibly CH2O†. Stable products as well as the visible luminescence due to the electronically excited diatomic radicals are followed as a function of pressure, time, and addition of the free radical scavenger gas NO. Both major photodecomposition routes appear to be non‐Boltzmann.

Unravelling the effects of radiation forces in water

The effect of radiation forces at the interface between dielectric materials has been a long-standing debate for over a century. Yet there has been so far only limited experimental verification in complete accordance with the theory. Here we measure the surface deformation at the air–water interface induced by continuous and pulsed laser excitation and match this to rigorous theory of radiation forces. We demonstrate that the experimental results are quantitatively described by the numerical calculations of radiation forces. The Helmholtz force is used for the surface radiation pressure. The resulting surface pressure obtained is consistent with the momentum conservation using the Minkowski momentum density expression assuming that the averaged momentum per photon is given by the Minkowski momentum. Considering the total momentum as a sum of that propagating with the electromagnetic wave and that deposited locally in the material, the Abraham momentum interpretation also appears to be appropriate.

Photothermal lens spectrometry of homogeneous fluids with incoherent white-light excitation using a cylindrical sample cell

A model for photothermal lens signal generation in a cylindri- cal sample cell under constant irradiance excitation is described and tested. The model is developed with and without the assumption that the sample cell does not change temperature over the irradiation time. In both cases, the photothermal lens is predicted to be parabolic in form with a strength that is independent of sample cell radius. The predicted irradiance independence suggests that incoherent illumination can be used to perform photothermal lens spectroscopy in low-volume cells. Experimental evidence is obtained using a Xe arc lamp to perform pho- tothermal lens spectroscopy in a 6 mL cylindrical spectrophotometric cell. Optical filters are used to reduce the power at IR and UV wavelengths of the Xe lamp emission spectrum. This pseudo-white-light source enables indirect optical absorbance measurement independent of the absorption spectrum of the analyte. The preliminary data reported show that photo- thermal lens signals can be obtained using wide-spectral-bandwidth, in- coherent excitation sources. Although the theoretical enhancement fac- tor is found to be only ;0.01 for these experiments, limits of detection of the order of 30 to 300 pM pseudoisocyanine dye in ethanol solution are found. This corresponds to spectral integrated absorption detection limit from 10 24 to 10 26 au in the centimeter path length cell. These low de- tection limits are found even with low enhancement factors because the factors that affect the noise in the photothermal lens and conventional transmission spectroscopy signals are not the same in these experi- ments. The major sources of uncertainty in these detection limit esti- mates are knowledge of the excitation source spectrum and periodic chaotic behavior of the diode laser used as a probe of the photothermal lens. Examination of the response time of the signal reveals that thermal conductivity of the sample cell influences the characteristic signal rise and decay time constants. The radiative heat transfer model is applied to interpret measured time constants in terms of the cell thermal conduc- tivity and thickness of the sample cell walls. The sample cell thermal conductivity determined by this method is consistent with ferrous mate- rials.

Accounting for absorption saturation effects in pulsed infrared laser-excited photothermal spectroscopy

Equations that relate photothermal lens focal lengths and photothermal deflection angles to saturation absorption coefficients are derived. These equations are derived for two-level absorbers with both homogeneously and inhomogeneously broadened transitions. Initial and time-dependent photothermal lens signals are calculated. Equations describing the zero-time signals are exact to within the simplifying assumptions of the derivation, while the time-dependent signals are approximate. The approximation is performed by the use of a finite series of Gaussian functions to model the temperature change profile distorted by nonlinear absorption. The excitation irradiance-dependent signal behavior for rectangular and exponential excitation pulse time profiles for homogeneously and inhomogeneously broadened transitions are compared. Absorbed energies are used to calculate effective absorbances obtained by the use of conventional and photothermal lensing spectrometry. The conclusions drawn from these comparisons are tha pulsed laser photothermal spectroscopy is sensitive to the excitationlasers pulse temporal profile and the transition broadening mechanism.

A quantitative test of unimolecular rate theory in the multiphoton dissociation of CF2CFCl

We have measured the distribution of total energy in reaction products for the CO2 laser‐induced MPD: CF2CFCl→CF2+CFC1. From a separate study of MPD rates as a function of laser intensity and inert buffer gas pressure, reliable estimates of the radiative pumping rates are known for this reaction. These results, when analyzed together, allow us to extract a unimolecular A factor from the MPD data. The determined value A = 3×1016 sec −1 agrees well with estimates based on independent thermal data.