Tunable Vacuum Ultraviolet Laser Research Articles

Energy partitioning in the femtosecond-laser-induced associativeD2desorption from Ru(0001)

Energy transfer to different degrees of freedom during the femtosecond-laser-induced recombinative desorption of ${\mathrm{D}}_{2}$ from a deuterium-covered Ru(0001) surface $({\mathrm{D}}_{\mathrm{ads}}+{\mathrm{D}}_{\mathrm{ads}}∕\mathrm{Ru}\ensuremath{\rightarrow}{\mathrm{D}}_{2,\text{gas}}+\mathrm{Ru})$ has been investigated. $(1+{1}^{\ensuremath{'}})$-resonance-enhanced multiphoton photoionization (REMPI) and time-of-flight (TOF) measurements are utilized to provide information on the internal and external (translational) energy content, respectively. Rovibrational population distributions of the reaction product are detected via various $B\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{u}^{+}\ensuremath{\leftarrow}X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+}$ Lyman bands using tunable vacuum ultraviolet laser radiation in the resonant excitation step. Rotational quantum state populations in the vibrational ground state and the first excited state are measured yielding average rotational energies of $⟨{E}_{\mathrm{rot}}⟩∕{k}_{\mathrm{B}}=800$ and $1500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively, for an absorbed laser fluence $⟨F⟩$ of $85\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$. In addition, a mean vibrational energy of the desorbing molecules is extracted which amounts to $⟨{E}_{\mathrm{vib}}⟩∕{k}_{\mathrm{B}}=1200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Extensive TOF measurements enable complete energy balancing with $⟨{E}_{\mathrm{trans}}⟩∕2{k}_{\mathrm{B}}=2500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $⟨F⟩=85\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$ and underline the nonthermal and unequal energy partitioning between the different degrees of freedom within the reaction product. The effects of multidimensional electronic friction between substrate and adsorbate layer and peculiarities of the potential energy landscape governing the ${\mathrm{D}}_{2}$ recombination are discussed.

High resolution study of Q-branches in the E1Π– X1Σ + (0,0) band of 12C 16O, 13C 16O, and 13C 18O

A tunable and narrowband vacuum ultraviolet laser system near λ=107.5 nm is used to record Doppler-free molecular beam spectra of the E 1Π– X 1Σ + (0,0) band for three isotopomers of carbon monoxide. The Q branches of the E– X (0,0) bands in 12C 16O, 13C 16O, and 13C 18O are fully resolved in this study, performed at a resolution of 0.02 cm −1 (FWHM). The newly obtained data are included in a comprehensive fitting routine with previous data to yield optimized molecular constants for the upper level.

Vacuum Ultraviolet Laser-Induced Fluorescence Detection of O(1S) Atom Produced in the 193 nm Photolysis of Ozone

The O(2p1S) atoms produced in the photolysis of O3 have been detected by a technique of laser-induced fluorescence spectroscopy with the tunable vacuum ultraviolet (VUV) laser radiation around 121.76 nm. The quantum yield value for O(1S) formation is determined to be (2.5 ± 1.1) × 10-3 in the 193 nm photolysis of O3, which is determined by comparing the VUV laser-induced fluorescence intensities of the O(3s1P1 − 2p1S0) transition with those of the H(2p2Pj ← 1s2S) transition at 121.56 nm of the H atoms generated from the photolysis of HCl at 193 nm. The O(1S) detection technique used in this study is very sensitive, and the detection limit is estimated to be 1 × 109 atoms cm-3. The contribution for the OH radical production from the reaction of H2O with O(1S) produced in the UV photolysis of O3 relative to that from the O(1D) + H2O reaction has been estimated as a function of altitude in the stratosphere, using the photolytic O(1S) quantum yield value obtained in this study and the O(1S) reaction rate coefficients reported previously. The maximum contribution of the O(1S) reaction to OH production rate is a 14% fraction of that from the O(1D) reaction at 40 km altitude at mid-latitudes, assuming the spin-forbidden dissociation process, O(1S) + O2(X3Σg-), for the formation of O(1S) in the photolysis of ozone. Importance of precise measurements of the temperature-dependent reaction rates for O(1S) has been suggested.

Measurements of atomic carbon density in processing plasmas by vacuum ultraviolet laser absorption spectroscopy

Measurements of the absolute C atom density in an inductively coupled plasma (ICP) source were carried out by using vacuum ultraviolet (VUV) laser absorption spectroscopy with the resonance lines of C atoms at wavelengths around 94.5 and 165.7 nm. A tunable VUV laser covering these wavelength ranges was generated by a two-photon resonance/four-wave mixing technique in Xe gas. No absorption at around 94.5 nm could be observed, but from the absorption spectra around 165.7 nm we successfully derived the absolute density of C atoms in the ICP source. The obtained values varied from 1×1010 to 1×1011 cm−3, depending on the source gas and operating conditions of the plasma source. The relatively small density values compared to other atomic species are attributed to the large loss rates, which mostly occur on the surface.

Vibrational and rotational population distribution ofD2associatively desorbing from Pd(100)

The rotational and vibrational state distribution of ${\mathrm{D}}_{2}$ molecules desorbing from Pd(100) is determined by tunable vacuum ultraviolet laser ionization spectroscopy. For the deuterium supply a permeation source is used operating at surface temperatures in the range from 400 to 850 K. The molecules are detected in rovibrational states up to ${(v}^{\ensuremath{''}}{,J}^{\ensuremath{''}})=(0,12)$ and (1,8). A significant rotational cooling is observed together with a strong vibrational heating. These experimental results are found to be in good agreement with quantum-mechanical calculations based on an ab initio potential-energy surface. A consistent microscopic picture of the hydrogen reaction dynamics on Pd(100) is established and thus a recent controversy about the application of the principle of detailed balance in the system ${\mathrm{D}}_{2}/\mathrm{Pd}(100)$ can be settled.

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Large discrepancies in the published photoabsorption oscillator strengths of the vacuum-ultraviolet (VUV) bands of CO hamper the interpretation of VUV astronomical observations. We report new oscillator strength measurements of the (0, 0) and (1, 0) bands of the CO B1Σ+-X1Σ+ electronic system determined from high-resolution (resolving power ≈ 750,000) absorption spectra recorded with a tunable VUV laser system. The instrumental bandwidth (~ 0.14 cm-1 FWHM) was significantly less than the widths of the Doppler-broadened CO lines (~ 0.20 cm-1 FWHM). The B(0)-X(0) and B(1)-X(0) band oscillator strengths, derived from measurements of the photoabsorption cross sections of 36 individual rotational lines, are (6.5 ± 0.6) × 10-3 and (1.1 ± 0.1) × 10-3, respectively. Our results are compared with other recent laboratory measurements and with ab initio calculations.

Dissociation dynamics of acetylene Rydberg states as a function of excited state lifetime

The state selective photodissociation of acetylene, C2H2/C2D2, was studied in the wavelength range 121.2–132.2 nm by high resolution Rydberg atom time-of-flight measurements on the atomic fragment, H/D. In the wavelength region studied members of all four Rydberg series and the highly excited Ẽ valence state were state selectively excited using tunable vacuum-ultraviolet laser radiation. The lifetime of the excited states which were studied varied from 58 fs to more than 2 ps. Formation of the ethynyl radical in its X̃ electronic ground state and its first electronically excited à state is observed with practically no indication of B̃ state fragments. Two decay channels with different dissociation dynamics were also observed. In both channels the observed decay dynamics depended strongly on the excited state of the parent molecule. Further there are major differences between these two dissociation pathways with respect to the measured internal energy and angular distributions. In one channel the dissociation is dominated by dynamical effects and the C2H fragments are formed with a high degree of vibrational excitation. In contrast to this in the second channel a smooth internal energy distribution is observed indicating that the fragment quantum state distribution is spread over a considerable range of the available phase space. Moreover, this second channel can be fit with a phase space model constrained only by conservation of energy and angular momentum. This is further evidence for the randomization of internal energy during the dissociation process.

High‐Resolution Oscillator Strength Measurements for High‐v′ Bands of theA1Π(v′)–X1Σ+(v″ = 0) System of Carbon Monoxide

Band oscillator strengths for six bands of the CO A1Π(v')-X1Σ+(v'' = 0) system with 13 ≤ v' ≤ 21 have been determined from high-resolution (resolving power ≈ 750,000) absorption spectra recorded with a tunable vacuum ultraviolet (VUV) laser system. The instrumental bandwidth of the system, ≈ 0.11 cm-1 FWHM, was significantly less than the widths of the Doppler-broadened CO lines (≈ 0.20 cm-1 FWHM). Integrated cross sections of individual rotational lines were determined by a least-squares fitting routine, taking into account the effects of the finite instrumental resolution on the measured absorption features. Our derived f-values for the A(13)-X(0), A(14)-X(0), and A(16)-X(0) bands are consistent with other recent measurements, with the ab initio electronic transition moment calculations of Kirby & Cooper, and with f-values derived from the ab initio electronic transition moment calculations of Spielfiedel & Feautrier cited by Jolly et al. Our f-values for the A(18)-X(0), A(20)-X(0), and A(21)-X(0) bands, the first directly measured values for these bands, progressively deviate from band oscillator strengths derived from the electronic transition moment function of Kirby & Cooper. In addition, a pressure-broadening coefficient of (3.1 ± 0.6) × 10-4 cm-1 torr-1 was determined for room-temperature CO-CO collisions.

Flame sampling photoionization mass spectrometry of dichloroethenol

A newly developed flame-sampling laser-ionization mass spectrometer has been used for the detection of combustion intermediates in a low-pressure methane/oxygen flame seeded with trichloroethylene. Among the most prominent intermediates are 1,1-dichloroethylene and 2,2-dichloroethenol. Near-threshold photoionization efficiency curves, produced with a tunable vacuum ultraviolet (VUV) laser light source, are used for isomerically selective detection of these species. The previously unmeasured ionization potential for 2,2-dichloroethenol is placed at 9.09±0.02 eV.

Flame sampling photoionization mass spectrometry of CH 3PO 2 and CH 3OPO 2

Laser photoionization mass spectrometry has provided the first detection of the isolated dioxophosphoranes, CH 3PO 2 and CH 3OPO 2. These molecules are found to be key intermediates in the combustion of dimethyl methylphosphonate (DMMP). Near-threshold photoionization efficiency curves, produced with a tunable vacuum ultraviolet (VUV) laser light source, yield ionization potentials of 9.24 ± 0.01 and 8.97 ± 0.02 eV for CH 3PO 2 and CH 3OPO 2, respectively. A newly developed flame-sampling laser-ionization mass spectrometer provided measurements of concentration profiles of these species throughout the flame zone of a premixed, low pressure hydrogen/oxygen/argon flame, seeded with DMMP.

Rotational Analyses for Selected Bands of the 2 ← X1 Transition of Cl2

Rotational analyses are presented for selected bands of the 21 (v‘= 1−3) ← X1 (v‘‘ = 0) transition of supersonically cooled Cl2, recorded in fluorescence excitation near λ ≤ 128 nm with a tunable, coherent, and monochromatic vacuum ultraviolet laser generated by four-wave difference mixing in Kr gas. Absolute J-numbering was established in part by spectral simulation. Although the band system is perturbed, the effective rotational constants derived in this work are consistent with theoretical calculations that place the excited two-state equilibrium bond length at ∼2.1 A.

Tunable vacuum ultraviolet laser spectroscopy of XeAr and XeNe near 68 000 cm−1: Interatomic potentials mediated by a 6s Rydberg electron

The laser induced fluorescence spectra of XeAr and XeNe have been measured near the Xe 6s[3/2]°1−1S0 transition at 68 045.663 cm−1. In XeAr, nine discrete peaks were observed, attributed to excitation to vibrational levels of the excited Ω=0+ electronic state, merging into a continuum. From a Franck–Condon analysis, the potential for the excited Ω=0+ state of XeAr was determined. It was found that this potential has a shallow minimum at long range [De′=28(1) cm−1, re′=5.44(7) Å] with a shallow repulsive wall. The shallow repulsive wall is attributed to the influence of the XeAr+ ion core at short range. The role of the 6s Rydberg electron of Xe in determining the shape of the interatomic potentials of excited XeRg (Rg=a rare gas atom) is discussed on the basis of a simple model potential, in which the interatomic potential is described as the sum of a diatomic ion core (XeRg+) potential term and a term representing the exchange repulsion between the Rydberg electron and the rare gas moiety. For XeNe, only continuum spectra were observed, indicating that the upper states are not bound.

Tunable vacuum ultraviolet laser — Ideal light for spectroscopy of atoms, molecules, and clusters

Application of a tunable, intense, and coherent vacuum ultraviolet (VUV) light source, a so-called tunable VUV laser, to molecular spectroscopy and photodissociation chemistry is introduced by referring to our recent studies on the high-lying electronically excited states of small jet-cooled molecules and van der Waals clusters. It is demonstrated that the combination of the VUV laser, generated by two-photon resonance four wave sum or difference frequency mixing scheme, and the free jet expansion techniques is promising to derive precise and substantial information regarding the level structure and the dynamical processes characteristic of highly excited molecular species excited in the VUV wavelength region ( λ=200-115 nm).

VUV photofragment excitation spectroscopy: Dynamics on dissociative potential surface of OCS

The measurements of the photofragment excitation (PHOFEX) spectrum of jet-cooled OCS were extended toward the 152.6 and 150.6 nm peaks in the 1∑ +- 1∑ + transition. The PHOFEX spectra were recorded by scanning the wavelength of a tunable vacuum ultraviolet (VUV) photolysis laser while probing the S( 1S) product by a second laser. Due to the efficient cooling of the sample in the free-jet expansion and the high-resolving power of the VUV laser, the homogeneous broadening of the dissociative state was derived directly from the observed PHOFEX spectrum. Based on the measured peak width, the lifetimes (τ) of the dissociative states at 152.6 and 150.6 nm were determined to be τ ≈ 50 and 31 fs. The PHOFEX peaks were found to exhibit an asymmetric line profile, and a fitting to a Fano line formula led to the asymmetry parameters, q = −8.1(2) and −13.0(6), for the 152.6 and 150.6 nm peaks, indicating a considerable mixing of the continuum and discrete states in that particular energy region. On the basis of the determined lifetimes and q values, together with the corresponding values for the 154.5 nm peak, τ ≈ 140 fs and q = −3.5(9), the dissociation dynamics on the repulsive 1∑ + surface was inferred.

Photofragment action spectroscopy of the acetylene molecule in the vacuum ultraviolet

The photodissociation dynamics of acetylene to hydrogen atoms, C 2H 2 → C 2H + H and C 2D 2 → C 2D + D, with C 2H and C 2D in the ground X ̃ 2 Σ + and/or excited electronic A ̃ 2 Π states, has been investigated for the first time with tunable narrow-band vacuum ultraviolet laser radiation between 121 and 133 nm by employing hydrogen atom photofragment action spectroscopy. Action spectra have been taken at 2.5 cm −1 resolution with acetylene in a molecular beam adiabatically cooled internally to about 4 K. They exhibit pronounced discrete predissociative resonances of Rydberg and valence excited states resolved to homogeneous widths. From them dissociation lifetimes of individual states from 5 × 10 −14 s to longer than 2 × 10 −12 s are derived.

Photodissociation and Vibrational Dynamics of OCS in the VUV Region

Abstract The photofragment excitation (PHOFEX) spectrum of jet-cooled OCS was remeasured for the 154.5 nm peak in the 1Σ+–1Σ+ transition by scanning a tunable vacuum ultraviolet (VUV) laser wavelength in the deep VUV region and by probing the S(1S) product with a second laser. Due to the efficient cooling of the sample in the free-jet expansion and the high-resolving power of the VUV laser, the homogeneous broadening of the dissociative state was derived directly from the observed PHOFEX spectrum. Based on the measured peak width, the lifetime (τ) of the dissociative state was determined to be τ 0.14 ps. The profile of the PHOFEX peak was found to be asymmetric and was fit to a Fano line profile to derive the value of the asymmetry parameter, q = −3.5(9), which indicates a considerable mixing of the continuum and discrete states in that particular energy region. A Fourier-transform analysis was also performed for the whole distinct absorption feature in the 160—140 nm region measured by McCarthy and Vaida [J. Phys. Chem., 92, 5875 (1988)], and the period of the vibrational motion on the dissociative potential-energy surface was derived to be 41 fs. Using these pieces of experimental evidence, the dissociation dynamics on the repulsive 1Σ+ surface in the 154.5 nm region was inferred.

The Ω=1 van der Waals and Ω=0+ double well potentials of Xe 6s[3/2]1+Kr 1S determined from tunable vacuum ultraviolet laser spectroscopy

The laser induced fluorescence spectrum of jet-cooled XeKr has been measured in the vicinity of the Xe 6s[3/2]01–1S0 atomic transition at 68 045.663 cm−1. The spectrum consists of two band systems, corresponding to transitions to the Ω=0+,1 electronic states from v″=0 of the ground electronic state. By using the observed band positions and intensities, we have constructed model potentials for both excited electronic states. The Ω=0+ state has a double minimum potential [inner well, re′ = 3.09(3) Å, De′ = 624(3) cm−1; outer well, re′ = 5.1(2) Å, De′ = 101(1) cm−1] while the Ω=1 state potential has only a shallow van der Waals potential [re′ = 5.24(4) Å, De′ = 52.2(7) cm−1]. The double minimum potential for the Ω=0+ state and the difference between the potentials for the Ω=0+ and Ω=1 states are understood in terms of the dominance of two different types of bonding interactions over different ranges of the internuclear distance. At long range, the interaction is dominated by weak dispersion and overlap repulsion between the closed shell Kr atom and the excited Xe atom, giving rise to shallow minima at r≊5 Å in both states. At short range, the XeKr interaction is better described by a XeKr+ ion-core with an excited 6sσ Rydberg electron. The Ω=0+ state is associated with the strongly bound 2Σ+1/2 XeKr+ ion-core, while the Ω=1 state corresponds to the weakly bound 2Π3/2 XeKr+ ion-core. The dual nature of the bonding which gives rise to the double minimum potential in the Ω=0+ state is similar to the bonding seen in excited states of HgAr and HgNe [Duval et al., J. Chem. Phys. 85, 6324 (1986); Okunishi et al., ibid. 98, 2675 (1993); Onda et al., ibid. 101, 7290 (1994); Onda and Yamanouchi, ibid. (submitted)] or the long range s–s, short range d–d bonding seen in the ground state of Cr2 [Casey and Leopold, J. Chem. Phys. 97, 816 (1993)], but is different from some double minima states seen in other diatomics, such as H2 (E,F 1Σ+g) [Davidson, J. Chem. Phys. 35, 1189 (1960); Kolos and Wolniewicz, ibid. 50, 3228 (1968)], Na2 (4 1Σ+g) [Tsai et al., J. Chem. Phys. 101, 25 (1994)], and Cl2 (1 1Σ+u) [Yamanouchi et al., Chem. Phys. Lett. 156, 301 (1989); Tsuchizawa et al., J. Chem. Phys. 93, 111 (1990)] which arise from curve crossings between ionic and covalent diabatic states.

Laser-induced fluorescence detection of ClO radicals at 167–180 nm

ClO radicals are detected by a laser-induced fluorescence technique, using the C 2Σ–X 2Π transition with a tunable vacuum ultraviolet laser system. Predissociation in the C 2Σ state is discussed in terms of the fluorescence lifetime and excitation spectra. The vibrational distribution of ClO produced by the reaction between O1D and HCl is measured.

Mass-resolved VUV laser spectra in the vicinity of the Rydberg minimum of the 1 1∑ +u state of Cl 2

Excitation spectra of the 1 1∑ u + ← X 1∑ g + transition of Cl 2 near 136 nm are presented. Jet-cooled molecules were excited with a tunable vacuum ultraviolet laser generated by four-wave difference-mixing in Kr, and the resultant photoions were mass discriminated in a time-of-flight spectrometer. Rotationally resolved transitions for vibrational levels localized in the Rydberg portion of the 1-state double well potential were recorded and analyzed for all three naturally occurring Cl 2 isotopomers.

Application of Tunable Vacuum-Ultraviolet Laser to Chemical Reaction Dynamics.

Detailed insight into the dynamics of chemical reactions, nonreactive collisions, and molecular photofragmentation processes is provided by measurements of the translational-andinternal-energy, and spacial angular distributions of the products. With a tunable vacuumultravioletlaser, it becomes appealing to study the chemical reaction dynamics involving lightatoms (hydrogen, oxygen, sulfur, chlorine) and molecules. In this article, we have describedrecent progress in the field of chemical reactions (translational energy dependence of reactivecross sections, lifetime of the intermediate complex, stereochemistry, site-specificity), nonreactivecollisions (velocity and spacial anisotropy relaxation, intersystem crossing, intramultiplettransition), photodissociation (site-specificity, primary processes, adiabaticity of potentialcurves, vector correlation), and nonadiabatic coupling among electronic states.