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We present a novel spectroscopic technique that exhibits high sensitivity and a large dynamic range for the measurement of absolute absorption coefficients. We perform a simultaneous and correlated laser-induced fluorescence and cavity ring-down measurement of the same sample in a single pulsed laser beam. The combined measurement offers a large dynamic range and a lower limit of detection than either technique on its own. The methodology, dubbed cavity-enhanced laser-induced fluorescence, is developed and rigorously tested against the electronic spectroscopy of 1,4-bis(phenylethynyl)benzene in a molecular beam and density measurements in a cell. We outline how the method can be used to determine absolute quantities, such as sample densities, absorption cross sections, and fluorescence quantum yields, particularly in spatially confined samples. [ABSTRACT FROM AUTHOR]

Using visible and x-ray photoelectron spectroscopy, we measured the work function of a Au(111) surface at a well-defined submonolayer coverage of Cs. For a Cs coverage producing a photoemission maximum with a He–Ne laser, the work function is 1.61±0.08 eV, consistent with previous assumptions used to analyze vibrationally promoted electron emission. A discussion of possible Cs layer structures is also presented. [ABSTRACT FROM AUTHOR]

DCl+(X2Π3/2,v+''=0)cations have been prepared by 2+1 resonance enhanced multiphoton ionization, and their subsequent fragmentation following excitation at numerous wavelengths in the range of 240–350 nm studied by velocity map imaging of the resulting Cl+ products. This range of excitation wavelengths allows selective population of A 2Σ+ state levels with all vibrational (v+′) quantum numbers in the range 0≤v+′≤15. Image analysis yields wavelength dependent branching ratios and recoil anisotropies of the various D+Cl+ (3PJ, 1D, and 1S) product channels. Levels with 10≤=v+′≤=15 have sufficient energy to predissociate, forming D+Cl+(3PJ) products with perpendicular recoil anisotropies—consistent with the A 2Σ+←X 2Π parent excitation and subsequent fragmentation on a time scale that is fast compared with the parent rotational period. Branching into the various spin-orbit states of the Cl+(3PJ) product is found to depend sensitively upon v+′ and, in the case of the v+′=13 level, to vary with the precise choice of excitation wavelength within the A 2Σ+←X 2Π(13,0) band. Such variations have been rationalized qualitatively in terms of the differing contributions made to the overall predissociation rate of DCl+(A,v+′) molecules by coupling to repulsive states of 4Π, 4Σ-, and 2Σ- symmetries, all of which are calculated to cross the outer limb of the A 2Σ+ state potential at energies close to that of the v+'= 10 level. Cl+(3PJ) fragments are detected weakly following excitation to A2Σ+ state levels with v=0 or 1, Cl+1D fragments dominate the ion yield when exciting via 2≤v+'≤6 and via v+'=9, while Cl+ (1D) fragments dominate the Cl+ images obtained when exciting via levels with v+'=7 and 8. Analysis of wavelength resolved action spectra for forming these Cl+ ions and of the resulting Cl+ ion images shows that (i) these ions all arise via two photon absorption processes, resonance enhanced at the one photon energy by the various A(v+'<10) levels, (ii) the first A2Σ+←X2π absorption step is saturated under the conditions required to observe significant two photon dissociation, and (iii) the final absorption step from the resonance enhancing A(v+') level involves a parallel transition. [ABSTRACT FROM AUTHOR]

We demonstrate that optical tweezers can be used to control and characterize the coagulation and mixing state of aerosols. Liquid aerosol droplets of 2–14 μm in diameter are optically trapped and characterized by spontaneous and stimulated Raman scatterings, which together provide a unique signature of droplet size and composition. From the conventional bright field image, the size of the trapped droplet can be estimated and compared with that determined from stimulated Raman scattering, and the motion of the particle within the trapping plane can be recorded. A maximum of four droplets can be manipulated in tandem by forming multiple optical traps through rapid beam steering. The coagulation of two droplets can be studied directly by controlling two droplets. The limiting conditions under which optical forces and capillary forces dominate the aerosol coagulation event are explored by varying the relative optical trap strengths and characterizing the coagulation of different droplet sizes. Finally, we demonstrate that the coagulation of different aerosol components can be compared and the mixing state of the final coagulated droplet can be investigated. In particular, we compare the outcome of the coagulation of an aqueous sodium chloride aerosol droplet with a second aqueous droplet, with an ethanol droplet or with a decane droplet. [ABSTRACT FROM AUTHOR]

Photodissociation experiments were carried out at 193 nm for single HCl molecules which are adsorbed on the surface of large Arn clusters and small (HCl)m complexes which are embedded in the interior of these clusters. For the surface case the size dependence is measured for the average sizes =140–1000. No cage exit events are observed in agreement with the substitutional position of the molecule deeply buried in the outermost shell. This result is confirmed by a molecular dynamics simulation of the pickup process under realistic conditions concerning the experiment and the interaction potentials. The calculations of the dissociation process employ the surface hopping model. For the embedded case the average sizes covered are =3 and 6 and =8–248. The kinetic energy of the H atom fragments is measured exhibiting peaks at zero and around 2.0 eV which mark completely caged and unperturbed fragments, respectively. The ratio of theses peaks strongly depends on the cluster size and agrees well with theoretical predictions for one and two closed icosahedral shells, in which the nonadiabatic coupling of all states was accounted for. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A set of photodissociation experiments and simulations of hydrogen iodide (HI) on Ar[sub n] clusters, with an average size =139, has been carried out for different laser polarizations. The doped clusters are prepared by a pick-up process. The HI molecule is then photodissociated by a UV laser pulse and the outgoing H fragment is ionized by resonance enhanced multiphoton ionization in a (2+1) excitation scheme within the same laser pulse at the wavelength of 243 nm. The measured time-of-flight spectra are transformed into hydrogen kinetic energy distributions. They exhibit a strong fraction of caged H atoms at zero-kinetic energy and peaks at the unperturbed cage exit for both spin–orbit channels nearly independent of the polarization. At this dissociation wavelength, the bare HI molecule exhibits a strict state separation, with a parallel transition to the spin–orbit excited state and perpendicular transitions to the ground state. The experimental results have been reproduced using molecular simulation techniques. Classical molecular dynamics was used to estimate the HI dopant distribution after the pick-up procedure. Subsequently, quasi-classical molecular dynamics (Wigner trajectories approach) has been applied for the photodissociation dynamics. The following main results have been obtained: (i) The HI dopant lands on the surface of the argon cluster during the pick-up process, (ii) zero-point energy plays a dominant role for the hydrogen orientation in the ground state of HI–Ar[sub n] surface clusters, qualitatively changing the result of the photodissociation experiment upon increasing the number of argon atoms, and, finally, (iii) the scattering of hydrogen atoms from the cage which originate from different dissociation states seriously affects the experimentally measured kinetic energy distributions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The fragmentation dynamics of allene and propyne molecules following photoexcitation at 193.3 nm and at 121.6 nm have been investigated by H(D) Rydberg atom photofragment translational spectroscopy. The total kinetic energy release (TKER) spectra of the H (and D) atoms resulting from H[sub 2]CCCH[sub 2], H[sub 3]CCCH, and D[sub 3]CCCH photolysis at 193.3 nm are found to be essentially identical. The results contradict conclusions reached in several previous studies of propyne photochemistry at this wavelength. The observed energy disposal, and the isomer independence, are most readily rationalized by assuming that the fragmentation of both molecules following excitation at 193.3 nm is preceded by internal conversion to the ground (S[sub 0]) state potential energy surface, and that the isomerization rate of the resulting highly vibrationally excited S[sub 0] molecules is faster than their unimolecular decay rate. The time-of-flight (TOF) and TKER spectra of the H and D atoms resulting from 121.6 nm photolysis of allene, propyne and propyne-d[sub 3] show significant differences, however. The differences can be reconciled by assuming two competing pathways for forming H(D) atoms following photoexcitation of propyne. One, involving selective cleavage of the acetylenic H[sub 3]CCC–H bond, is assumed to occur from the excited electronic state prepared by photon absorption or from a recognizably “propynelike” state to which it couples efficiently. The other, which yields a slower distribution of H(D) atoms, is considered to arise via radiationless transfer to a lower electronic state, isomerization, and subsequent unimolecular decay. The TOF and TKER spectra of the H atoms resulting from 121.6 nm photolysis of allene are indistinguishable from those associated with this second, “statistical” fragmentation channel in propyne. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the production in the gas phase of ionically bound HXeI molecules. The molecules are generated by the photodissociation of HI molecules in large Xe[SUBn] clusters and are identified from the asymmetry of the detected H atom fragments arising from the dissociation of oriented HXeI. The orientation, resulting from a synergistic action of a pulsed laser field with a weak electrostatic field, is quite pronounced, due to a large ratio of the polarizability anisotropy to the rotational constant of HXeI. [ABSTRACT FROM AUTHOR]

We report the first production of the molecule HXeI, which is bound by ionic forces, in the gas phase. The molecule is generated by the photodissociation of HI molecules on a large Xe[sub n] cluster and is identified by detecting the asymmetric distribution of the H atom fragments of the oriented HXeI. The orientation is achieved in a combined pulsed laser and weak electrostatic field making use of the large anisotropy in the polarizability and the large dipole moment of this molecule. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ultraviolet (UV) photodissociation experiments are carried out for Ar[sub n](HBr) clusters in which the HBr is adsorbed on the surface of the Ar[sub n], and also on isomers of these systems in which HBr is embedded within the rare-gas cluster. The mean size of the cluster distribution in the experiments is around n¯=130. The kinetic energy distribution (KED) of the hydrogen atoms that left the clusters is measured. Molecular dynamics (MD) simulations of the photodissociation of the chemically similar clusters Ar[sub n](HCl) are used to provide a qualitative interpretation of the experimental results. The clusters with embedded HBr give a very cold H-atom KED. The clusters with the surface-adsorbed HBr give a KED with two peaks, one corresponding to very low energy H atoms and the other pertaining to high energies, of the order of 1.35 eV. The theoretical simulations show that already for n=54, there is a strong cage effect for the "embedded" molecule case, resulting in slow H atoms. The surface-adsorbed case is interpreted as due to two types of possible adsorption sites of HX on Ar[sub 55]: for a locally smooth adsorption site, the cage effect is relatively weak, and hot H atoms emerge. Sites where the HBr is adsorbed at a vacancy of Ar[sub n] lead to "encapsulation" of the H atom produced, with a strong cage effect. A weak tail of H atoms with energies well above the HBr monomer excess energy is observed for the embedded case. Simulations support that this is due to a second photon absorption by recombined, but still vibrationally hot, HBr. The results throw light on the differences between the cage effect inside bulk structure and at surfaces. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]