Resonant two-photon ionization spectroscopy of jet-cooled OsC

The optical spectrum of diatomic OsC has been investigated for the first time, with transitions recorded in the range from 17 390 to 22 990 cm(-1). Six bands were rotationally resolved and analyzed to obtain ground and excited state rotational constants and bond lengths. Spectra for six OsC isotopomers, 192 Os 12C (40.3% natural abundance), 190 Os 12C(26.0%), 189 Os 12C(16.0%), 188 Os 12C(13.1%), 187 Os 12C(1.9%), and 186 Os 12C(1.6%), were recorded and rotationally analyzed. The ground state was found to be X 3 Delta 3, deriving from the 4 delta 3 16 sigma 1 electronic configuration. Four bands were found to originate from the X 3 Delta 3 ground state, giving B 0"=0.533 492(33) cm(-1) and r 0 "=1.672 67(5) A for the 192 Os 12C isotopomer (1 sigma error limits); two of these, the 0-0[19.1]2<--X 3 Delta 3 and 1-0[19.1]2<--X 3 Delta 3 bands, form a vibrational progression with Delta G' 1/2=953.019 cm(-1). The remaining two bands were identified as originating from an Omega"=0 level that remains populated in the supersonic expansion. This level is assigned as the low-lying A 3 Sigma 0+ (-) state, which derives from the 4 delta 2 16 sigma 2 electronic configuration. The OsC molecule differs from the isovalent RuC molecule in having an X 3 Delta 3 ground state, rather than the X 2 delta 4, 1 Sigma+ ground state found in RuC. This difference in electronic structure is due to the relativistic stabilization of the 6s orbital in Os, an effect which favors occupation of the 6s-like 16 sigma orbital. The relativistic stabilization of the 16 sigma orbital also lowers the energy of the 4 delta 2 16 sigma 2, 3 Sigma(-) term, allowing this term to remain populated in the supersonically cooled molecular beam.     

Resonant two-photon ionization spectroscopy of jet-cooled PdSi

The spectra of diatomic PdSi have been investigated for the first time, using the technique of resonant two-photon ionization spectroscopy. A number of vibronic transitions have been observed in the 20,400-22,000 cm(-1) range. It is difficult to group the bands into band systems, although one likely band system has been identified. Three bands have been rotationally resolved and analyzed, two of which are perturbed by interactions with other states. The data show that the ground state of PdSi has Ω = 0, and a bond length of r(0)(') = 2.0824(3) A?. Comparisons to previously published density functional theory calculations provide strong support for the assignment of the ground state to the 1σ(2) 2σ(2) 1π(4) 1δ(4) 3σ(2), (1)Σ(+) term, which is predicted to be the ground state in the calculations. The much shorter bond length and greater bond energy of PdSi, as compared to its isoelectronic counterpart, AlAg, demonstrate that there is strong π bonding in PdSi, as has been previously found for the other nickel group silicides, NiSi and PtSi.     

Resonant two-photon ionization spectroscopy of jet-cooled OsN: 520-418 nm

The optical transitions of supersonically cooled OsN have been investigated in the range from 19,200 to 23,900 cm(-1) using resonant two-photon ionization spectroscopy. More than 20 vibronic bands were observed, 17 of which were rotationally resolved and analyzed. The ground state is confirmed to be (2)Δ(5/2), deriving from the 1σ(2) 2σ(2) 1π(4) 1δ(3) 3σ(2) electronic configuration. The X (2)Δ(5/2) ground state rotational constant for (192)Os(14)N was found to be B(0) = 0.491921(34) cm(-1), giving r(0) = 1.62042(6) ? (1σ error limits). The observed bands were grouped into three band systems with Ω' = 7/2 and four with Ω' = 3/2, corresponding to the three (2)Φ(7/2) and four (2)Π(3/2) states expected from the 1σ(2) 2σ(2) 1π(4) 1δ(3) 3σ(1) 2π(1) and 1σ(2) 2σ(2) 1π(4) 1δ(2) 3σ(2) 2π(1) electronic configurations. In addition, two interacting upper states with Ω' = 5/2 were observed, one of which is thought to correspond to a 1σ(2) 2σ(2) 1π(3) 1δ(3) 3σ(2) 2π(1), (2)Δ(5/2) state. Spectroscopic constants are reported for all of the observed states, and comparisons to related molecules are made. The ionization energy of OsN is estimated as IE(OsN) = 8.80 ± 0.06 eV.     

Laser two-photon ionization spectroscopy of molecules in liquids

A two-photon lonization technique has been developed and applied to determine the photoionization threshold of a molecule in liquid solution. The photoionization of pyrene in n-pentane was studied by monitoring the photocurrent of an≈ 10^?6 M solution as a function of the laser wavelength in the region 360–530 nm, corresponding to two-photon, transition in the region 180–265 nm. The photoionization threshold thus determined is 4.80 ± 0.02 eV. Resonances were observed in the two-photon ionization spectrum which are tentatively ascribed to preionization of one-photon forbidden transitions to states energetically degenerate with the continuum, reached via two-photon absorption.     

Laser induced fluorescence and ionization spectroscopy: Theoretical and analytical considerations for pulsed sources

Summary Several theoretical considerations are presented for the single resonance (single step) and double resonance (double step) techniques of fluorescence and ionization spectroscopy in flames and plasmas with tunable pulsed dye laser excitation. For short (several nanoseconds) laser pulses, single step excitation with one laser beam followed by collisional ionization can be useful from an analytical point of view even though ionization yields are low because of the short irradiation time. When two lasers, tuned to two connected atomic transitions, are spatially and temporally coincident in the atom cell, the ionization efficiency is greatly enhanced making the technique extremely sensitive, down to pg/ml levels. Atomic fluorescence suffers from losses due to such enhanced ionization process, the losses being more severe the closer the final excited level is to the ionization continuum. It is then shown that double resonance ionic fluorescence in an inductively coupled plasma is more suitable if high sensitivity needs to be achieved. The experimental results were obtained with an excimer laser (XeCl) which simultaneously pumped two dye lasers. The fluorescence and ionization techniques are also compared in terms of the potential interferences expected in the analysis of real samples.

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Laserinduzierte Fluorescenz- und Ionisationsspektroskopie: Theoretische und analytische Betrachtungen über gepulste Quellen

Zusammenfassung Es werden verschiedene theoretische Betrachtungen über die Einfach- und Doppelresonanzverfahren der Fluorescenz- und Ionisationsspektroskopie in Flammen und Plasmen (über Ein- bzw. Stufenprozeß) mit Anregung durch gepulste durchstimmbare Farbstofflaser vorgestellt. Für Laserimpulse von mehreren Nanosekunden Dauer kann die Einstufenanregung (Laseranregung mit nachfolgender Stoßionisation) vom analytischen Standpunkt durchaus nützlich sein, auch wenn die Ionisierungsausbeuten aufgrund der kurzen Bestrahlungszeit gering sind. Der Wirkungsgrad der Ionisierung kann beträchtlich erhöht werden, wenn zwei Laser, die auf miteinander gekoppelte atomare Übergänge abgestimmt wurden, in der Atomisierungszelle räumlich und zeitlich zusammentreffen; hierdurch wird das Verfahren äußerst empfindlich, bis in den pg/ml-Bereich.Die Einbußen, die die Atomfluorescenz aufgrund solcher Ionisationsprozesse erleidet, sind dabei um so größer, je näher der angeregte Endzustand zum Ionisationskontinuum liegt. Wie weiterhin gezeigt wird, ist es für eine hohe Empfindlichkeit zweckmäßiger, Doppelresonanz-Ionenfluorescenz in einem induktiv gekoppelten Plasma anzuregen. Die experimentellen Ergebnisse wurden mit einem Excimerlaser (XeCl) erhalten, der gleichzeitig zwei Farbstofflaser pumpte. Fluorescenz- und Ionisationsverfahren werden auch im Hinblick auf mögliche Interferenzen verglichen, die bei der Analyse realer Proben zu erwarten sind.

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This paper is an extension of the lecture given by one of the authors (N.O.) at the XXIV. Colloquium Spectroscopicum Internationale

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On leave from: Department of Chemistry, University of Florida, Gainesville, Florida 32611, USA     

Site-specific H/D exchange of p-methoxyphenol studied by resonant two-photon ionization and mass-analyzed threshold ionization spectroscopy

The origin of the S(1) <-- S(0) transition (E(1)) and the adiabatic ionization energy (IE) of cis-p-methoxyphenol-d(1)-OD are determined to be 33 660 and 62 302 cm(-1), whereas those of cis-p-methoxyphenol-d(1)-OCH(2)D are 33 669 and 62 323 cm(-1), respectively. Similarly, the E(1) and IE of trans-p-methoxyphenol-d(1)-OD are determined to be 33 563 and 62 191 cm(-1) and those of trans-p-methoxyphenol-d(1)-OCH(2)D are 33 575 and 62 216 cm(-1), respectively. Comparing these data with those of p-methoxyphenol suggests that the H/D exchange on the OH substituent gives rise to a red shift in both the E(1) and IE, whereas that on the OCH(3) group yields a blue shift. The mass-analyzed threshold ionization spectra of the selected isomers can be used as the fingerprints for molecular identification. Analysis of these cation spectra shows that the substituent-sensitive in-plane C-OH and C-OCH(3) bending (mode 9b) and breathing (mode 1) vibrations are active for all of these isomeric cations.     

Vacuum-ultraviolet ionization spectroscopy of the jet-cooled RNA-base uracil

Adiabatic ionization potential (9.3411 +/- 0.0008 eV) and cationic vibrational structure of the jet-cooled RNA-base uracil are both accurately and precisely determined for the first time using a vacuum-ultraviolet mass-analyzed threshold ionization spectroscopy.     

Picosecond resonant two-photon ionization of cold sodium clusters

Large sodium clusters, up ton=21, generated by a low-temperature modification of the laser ablation gas jet source were ionized with two photons of visible radiation from an amplified picosecond dye-laser system.     

Resonant two-photon ionization spectroscopy of BNB

Triatomic BNB has been produced by laser ablation of a boron nitride rod in a supersonic expansion of helium carrier gas and has been investigated using resonant two-photon ionization spectroscopy in the visible region. The B 2Pi(g)-X 2Sigma(u)+ band system has been recorded near 514 nm and is dominated by a strong origin band, which has been rotationally resolved and analyzed. Both the (11)B(14)N(11)B (64% natural abundance) and the (10)B(14)N(11)B (32% natural abundance) isotopic modifications have been analyzed, leading to the spectroscopic constants (and their 1sigma error limits) of B0"(X 2Sigma(u)+)=0.466 147(70), B0'(B 2Pi(g))=0.467 255(75), and A0'(B 2Pi(g))=6.1563(38) cm(-1) for (10)B(14)N(11)B, corresponding to r(B-N)"(X 2Sigma(u)+)=1.312 47(10) A and r(B-N)'(B 2Pi(g))=1.310 92(11) A. Very similar values are obtained for the more abundant isotopomer, (11)B(14)N(11)B: B0"(X 2Sigma(u)+)=0.444 493(69), B0'(B 2Pi(g))=0.445 606(70), A0'(B 2Pi(g))=6.1455(38) cm(-1), corresponding to r(B-N)"(X 2Sigma(u)+)=1.312 41(10) A and r(B-N)'(B 2Pi(g))=1.310 77(10) A. These results are discussed as they relate to Walsh's rules and are compared to results for related molecules.     

Quantum-chemical and correlation study of the tautomerism and ionization of 1,2,3-Trihydroxy-9,10-anthraquinone

1,2,3-Trihydroxy-9,10-anthraquinone (anthragallol) exists as an equilibrium mixture of the 9,10-, 2,9-, 1,2-, and 2,3-quinoid tautomers. Its anion was detected in the 9,10-, 1,10-, 2,9-, and 2,3-quinoid forms, and its metal complexes, the 9,10-and 2,9-quinoid forms. Ionization and complexation of anthragallol can shift the tautomeric equilibrium.     

Laser fluorescence excitation band profiles of jet-cooled tropolone

The fluorescence excitation spectrum of the A¹B₂−X¹A₁ transition of tropolone was investigated as a function of pulsed laser intensity, I. For 0.5 < I < 4.5 MW/cm² the weak fluorescence intensity was found to increase approximately as aI + bI². The band profiles, which showed no evidence of resolvable rotational fine structure, were modeled by computations assuming a Lorentzian lineshape for the individual rotational transitions.     

Laser induced fluorescence spectroscopy of NC3O

Laser induced fluorescence spectra of the NC(3)O radical in a supersonic jet have been observed. The radical was produced in a pulsed electric discharge of HC(3)N and O(2) diluted to 0.3% with Ar. A total of 17 vibronic bands with a radiative lifetime of approximately 30 ns have been observed in a region from 27 000 to 27 500 cm(-1). The observed vibronic bands are classified as (2)Pi(12)-(2)Pi(12), (2)Pi(32)-(2)Pi(32), and (2)Sigma-(2)Sigma types. The upper states of the (2)Sigma-(2)Sigma bands have large spin-rotation constants, which should be denoted as Sigma((+)) and Sigma((-)). From high-level ab initio calculations and rotational analyses, the observed transition was assigned to the B (2)Pi-X (2)A(") transition. Dispersed fluorescence spectra from the upper (2)Sigma and (2)Pi vibronic levels have also been observed, yielding fundamental vibrational frequencies for the nu(1), nu(2), nu(3), and nu(7) modes of the ground state.     

Laser induced stepwise and two-photon ionization studies of strontium in the air acetylene flame

Using strontium atoms present as a trace constituent in an air-acetylene flame as an example, a rich laser enhanced ionization spectrum was obtained. One pulsed tunable dye laser was tuned to a transition originating from the ground state (460.73 nm) and another scanned over different spectral regions. The lines obtained were spectroscopically characterized as to the type of absorption process, which included non-resonant processes as well as single wavelength, two-wavelength, and two-photon resonant processes.With a maximum irradiance of 100 MW cm⁻², two-photon transitions resulting in collisionally assisted ionization included the 5sns and 5dnd Rydberg series (up to 37s and 15d) together with a strong auto-ionizing transition at 431.10 nm. The complexity of the observed ionization spectrum when the irradiance is high indicates that spectral interferences need to be carefully considered in analytical applications.     

Two-color resonantly enhanced multiphoton ionization and zero-kinetic-energy photoelectron spectroscopy of jet-cooled indan

We report studies of supersonically cooled indan using two-color resonantly enhanced multiphoton ionization and two-color zero-kinetic-energy photoelectron spectroscopy. With the aid of ab initio and density-functional calculations, vibrational modes of the first electronically excited state of the neutral species and those of the cation have been assigned, and the adiabatic ionization energy has been determined to be 68458 +/- 5 cm(-1). Similar to the ground state and the first electronically excited state of the neutral molecule, the ground state of the cation is also proven to be nonplanar, with an estimated barrier of 213 cm(-1) and a puckering angle of 15.0 degrees. These conclusions will be discussed in comparison with a previous study of an indan derivative 1,3-benzodioxole.     

Laser spectroscopy and dynamics of the jet-cooled AsH2 free radical

The A (2)A(1)-X (2)B(1) electronic transition of the jet-cooled AsH(2) free radical has been studied by laser-induced fluorescence (LIF), wavelength-resolved emission, and fluorescence lifetime measurements. The radical was produced by a pulsed electric discharge through a mixture of arsine (AsH(3)) and high pressure argon at the exit of a pulsed valve. Nine vibronic bands were identified by LIF spectroscopy in the 505-400 nm region, including a long progression in the bending mode and two bands (1(0) (1) and 1(0) (1)2(0) (1)) involving the excited state As-H symmetric stretch. Single vibronic level emission spectra showed similar activity in the bending and symmetric stretching frequencies of the ground state. High-resolution spectra of the 0(0) (0) band exhibited large spin splittings and small, resolved arsenic hyperfine splittings, due to a substantial Fermi contact interaction in the excited state. The rotational constants obtained in the analysis gave effective molecular structures of r"(0)=1.5183(1) A, theta"(0)=90.75(1) degrees and r'(0)=1.4830(1) A, theta'(0)=123.10(2) degrees . The excited state fluorescence lifetimes vary dramatically with rovibronic state, from a single value of 1.4 micros to many with lifetimes less than 10 ns, behavior which the authors interpret as signaling the onset of a predissociative process near the zero-point level of the ground state.     

Multiphoton ionization and two-photon excitation spectroscopy of nitric oxide

The multiphoton ionization and two-photon excited fluorescence action spectra of nitric oxide are compared. At low fluence the two spectra differ when more than three photons are required for ionization. This may be due to the presence of several energy acquisition routes, providing new spectroscopic information on intermediate slates.     

Near-UV resonant two-photon ionization spectroscopy of gas phase guanine: evidence for the observation of three rare tautomers

In light of a recently published study on the IR spectroscopy of guanine in He droplets (Choi, M. Y.; Miller, R. E. J. Am. Chem. Soc. 2006, 128, 7320), the present letter proposes a new interpretation of the resonant two-photon ionization (R2PI) experiments on gas phase guanine, which is supported by quantum chemistry calculations. Whereas He droplet experiments detect the most stable forms, only one of these forms is observed (very marginally) in the R2PI spectrum, which is actually dominated by three less stable "rare" tautomers, whose stabilities lie in the 3-7 kcal/mol range. The absence of the most stable forms in the R2PI spectrum suggests that a tautomer-dependent ultrafast relaxation process takes place in the excited state of these stable tautomers. The present reinterpretation modifies qualitatively the picture of the excited state of guanine tautomers and should contribute to the understanding of the deactivation mechanisms taking place in the excited state of DNA bases.