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.     

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 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.     

One- and two-photon singlet oxygen generation with new fluorene-based photosensitizers.

The spectral properties of new fluorene-based photosensitizers for efficient singlet oxygen production are investigated at room temperature and 77 K. Two-photon absorption (2PA) cross-sections of the fluorene derivatives are measured by the open aperture Z-scan method. The quantum yields of singlet oxygen generation under one- and two-photon excitation (phi(delta) and 2PAphi(delta), respectively), are determined by the direct measurement of singlet oxygen luminescence at approximately 1270 nm. The values of phi(delta) are independent of excitation wavelength, ranging from 0.6-0.9. The singlet oxygen quantum yields under two-photon excitation are 2PAphi(delta) approximately 1/2 phi(delta), indicating that the two processes exhibit the same mechanism of singlet oxygen production, independent of the mechanism of photon absorption.     

Resonant two-photon ionization spectra of fluorotoluene–argon complexes

The S₁←S₀ transitions of three jet-cooled fluorotoluene–argon complexes are studied by the resonant two-photon ionization technique coupled with time-of-flight mass spectrometry. Detailed spectral analysis assisted by model calculations has allowed us to determine the stretching vibrational frequencies and two bending vibrational frequencies of the complexes. The results show that, the relative positions of the substituents (i.e., F and CH₃) have very little influence on the stretching vibrations of the complexes but strongly affect their bending vibrations. In addition, the Ar atom is found to influence the internal rotation of the methyl group in the complexes.     

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.     

Synthesis and characterization of water-soluble phenylene-vinylene-based singlet oxygen sensitizers for two-photon excitation

[reaction: see text] The synthesis and characterization of water-soluble singlet oxygen sensitizers with a phenylene-vinylene motif is presented. The principal motivation for this study was to better understand specific features of a water-soluble molecule that influence the photosensitized production of singlet oxygen upon nonlinear, two-photon excitation of that molecule. To achieve water solubility, sensitizers were synthesized with ionic as well as nonionic substituents. In the ionic approach, salts of N-methylated pyridine, benzothiazole, and 1-methyl-piperazine moieties were used, as were aryl-substituted sulfonic acid moieties. In the nonionic approach, aryl-substituted triethylene glycol moieties were used. Selected photophysical properties of the compounds synthesized were determined, including singlet oxygen quantum yields. Of the molecules examined, the most efficient singlet oxygen sensitizers had triethylene glycol units as the functional group that imparted water solubility. Molecules containing the ionic moieties did not make singlet oxygen in appreciable yield nor did they efficiently fluoresce. Rather, for these latter molecules, rapid charge-transfer-mediated non-radiative processes appear to dominate excited state deactivation.     

Importance of properties of the lowest and higher singlet excited states on the resonant two-photon ionization of stilbene and substituted stilbenes using two-color two-lasers

Radical cations of trans-stilbene and substituted trans-stilbenes (stilbenes and the radical cations denote Sand S(*+), respectively) were generated from the resonant two-photon ionization (TPI) in acetonitrile with irradiation of one-laser (266- or 355-nm laser) and with simultaneous irradiation of two-color two-lasers (266- and 532-nm or 355- and 532-nm lasers) with the pulse width of 5 ns each. The formation yields of S(*+), the TPI efficiency, depended on the properties of S in the lowest and higher singlet excited state (S(S(1)) and S(S(n))), generated from one-photon excitation with 266- or 355-nm laser and from two-photon excitation with simultaneous irradiation of 266- and 532-nm or 355- and 532-nm lasers, respectively. The TPI efficiency using two-color two-lasers increased compared with that using one-laser. It is confirmed that the TPI proceeds through two-step two-photon excitation with the S(0) --> S(1) --> S(n)() transition. In addition to the electronic character of S(S(0)) which depends on the substituent of S, oxidation potential, and molar absorption coefficient of the S(0) --> S(1) absorption as well-known important factors for the TPI efficiency, it is shown that properties of S(S(1)) and S(S(n)) such as lifetimes, electronic characters of S(S(1)) and S(S(n)), molar absorption coefficient of the S(1) --> S(n) absorption, and ionization rate from S(S(n)) are also important.     

Resonant two-photon ionization spectroscopy of C6H6-(SF6)1,2

Vibrationally resolved electronic spectra of heteroclusters C₆H₆-SF₆ and C₆H₆-(SF₆)₂ were studied in the spectral regions near the S₀-S₁, 0 ₀ ⁰ and 6 ₀ ¹ transitions of the benzene monomer. A nonvanishing 0 ₀ ⁰ vibrational band has been observed for C₆H₆-SF₆ with a C_3v symmetry. For both clusters we have determined the ionization potentials as well as the binding energies in the electronic ground state and the ionization state. The fragmentation of larger clusters (C₆H₆)_n(SF₆)_m is restricted to the loss of SF₆ molecules while the emission of C₆H₆ molecules have not been observed.     

One- and two-photon photosensitized singlet oxygen production: characterization of aromatic ketones as sensitizer standards

Singlet molecular oxygen, O2(a1Deltag), can be efficiently produced in a photosensitized process using either one- or two-photon irradiation. The aromatic ketone 1-phenalenone (PN) is an established one-photon singlet oxygen sensitizer with many desirable attributes for use as a standard. In the present work, photophysical properties of two other aromatic ketones, pyrene-1,6-dione (PD) and benzo[cd]pyren-5-one (BP), are reported and compared to those of PN. Both PD and BP sensitize the production of singlet oxygen with near unit quantum efficiency in a nonpolar (toluene) and a polar (acetonitrile) solvent. With their more extensive pi networks, the one-photon absorption spectra for PD and BP extend out to longer wavelengths than that for PN, thus providing increased flexibility for sensitizer excitation over the range approximately 300-520 nm. Moreover, PD and BP have much larger two-photon absorption cross sections than PN over the range 655-840 nm which, in turn, results in amounts of singlet oxygen that are readily detected in optical experiments. One- and two-photon absorption spectra of PD and BP obtained using high-level calculations model the salient features of the experimental data well. In particular, the ramifications of molecular symmetry are clearly reflected in both the experimental and calculated spectra. The use of PD and BP as standards for both the one- and two-photon photosensitized production of singlet oxygen is expected to facilitate the development of new sensitizers for application in singlet-oxygen-based imaging experiments.     

Theoretical prediction of new dipole-bound singlet states for anions of interstellar interest

Anions that exhibit dipole-bound singlet states have been proposed as a potential class of molecules that may be identified in the interstellar medium. Using high-level coupled cluster theory, we have computed the dipole moments, electron binding energies, and excited states of 14 neutral radicals and their corresponding closed-shell anions. We have calibrated our methods against experimental data for CH(2)CN(-) and CH(2)CHO(-) and demonstrated that coupled cluster theory can closely reproduce experimental dipole moments, electron binding energies, and excitation energies. Using these same methods, we predict the existence of dipole-bound excited states for six of the 14 previously unknown anions, including CH(2)SiN(-), SiH(2)CN(-), CH(2)SiHO(-), SiN(-), CCOH(-), and HCCO(-). In addition, we predict the existence of a valence-bound excited state of CH(2)SiN(-) with an excitation wavelength near 589 nm.     

Assessment of water-soluble pi-extended squaraines as one- and two-photon singlet oxygen photosensitizers: design, synthesis, and characterization

Singlet oxygen sensitization by organic molecules is a topic of major interest in the development of both efficient photodynamic therapy (PDT) and aerobic oxidations under complete green chemistry conditions. We report on the design, synthesis, biology, and complete spectroscopic characterization (vis-NIR linear and two-photon absorption spectroscopy, singlet oxygen generation efficiencies for both one- and two-photon excitation, electrochemistry, intrinsic dark toxicity, cellular uptake, and subcellular localization) of three classes of innovative singlet oxygen sensitizers pertaining to the family of symmetric squaraine derivatives originating from pi-excessive heterocycles. The main advantage of pi-extended squaraine photosensitizers over the large number of other known photosensitizers is their exceedingly strong two-photon absorption enabling, together with sizable singlet oxygen sensitization capabilities, for their use at the clinical application relevant wavelength of 806 nm. We finally show encouraging results about the dark toxicity and cellular uptake capabilities of water-soluble squaraine photosensitizers, opening the way for clinical small animal PDT trials.     

Resonant two-photon ionization spectra of o,p, m-xylene...Ar n (n = 1, 2)

The resonant two-photon ionization technique (R2PI) is used to study jet-cooled van der Waals (vdW) complexes o, p, m-xylene...Ar _n (n = 1, 2) through the S ₁ ← S ₀ transition around the origin band. We have tentatively assigned the main spectral features of these vdW complexes. The transitions of these complexes are red-shifted from those of the monomers. The influence of the substituent positions on the vdW vibrations is investigated.     

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.     

Resonant and Off-resonance ionization of laser-excited sodium vapor

Electrons produced in a sodium effusive beam irradiated by pulsed laser light have been energy-analyzed. For resonant excitation of the atoms to the 3p state, we have studied associative ionization of excited Na(3p) (AI), Penning ionization (PI) and photoionization of highly-excited Na(n l),n l=3d, 4p, 5s, 4d/4f, by measurements of electron energy spectra. Our interpretation of the spectra is supported by a numerical analysis of the data which leads to estimates of the cross sections compatible with values previously measured in other experiments. Off-resonance ionization is considered at a particular wavelength. It is presumed to involve laser-excited dimers Na ₂ ^** . Spectra show that highly-excited atoms are produced by photodissociation of Na ₂ ^** or from Na ₂ ^** -Na collisional energy transfer, and that these atoms support the dominant off-resonance ionization channel in our laser excitation conditions.     

Monoexcited singlet states and conformation of some acylpyridines

Substituent effects on the UV-spectra of formyl-, acetyl- and benzoylpyridines are investigated. The UV-bands are discussed by comparison with the calculated energies and oscillator strengths of all planar and twisted 2-acyl-, 3-acyl- and 4-acyl derivatives using the SCF CI-1 method. The structure of each ¹π^* ← ¹π transition is determined from the contributions of the individual MO transitions and their LCAO characteristics. The conformation of acylpyridines is discussed.     

Low-lying singlet and triplet electronic states of RhB

The low-lying XSigma+, a3Delta, A1Delta, b3Sigma+, B1Pi, c3Pi, C1Phi, D1Sigma+, E1Pi, d3Phi, and e3Pi electronic states of RhB have been investigated at the ab initio level, using the multistate multiconfigurational second-order perturbation (MS-CASPT2) theory, with extended atomic basis sets and inclusion of scalar relativistic effects. Among the eleven electronic states included in this work, only three (the X1Sigma+, D1Sigma+, and E1Pi states) have been investigated experimentally. Potential energy curves, spectroscopic constants, dipole moments, binding energies, and chemical bonding aspects are presented for all electronic states.     

Half-projected and projected Hartree-Fock calculations for singlet ground states. II. Lithium hydride

The half-projected Hartree–Fock function (HPHF ) for singlet states is defined as a linear combination of two Slater determinants which contains only spin eigenstates with even spin quantum numbers. The possible uses of such an approach for determining molecular properties are investigated computing the potential energy curve, binding energy, force constant, and dipole moment variation corresponding to the lithium hydride ground state. Full projected and restricted Hartree–Fock calculations (PHF and RHF ) are performed simultaneously for comparison purposes. It is found that the HPHF model yields very satisfactory results, very close to those of the PHF scheme. Both models predict properly the molecular behavior as a function of nuclear separation, whereas the RHF one fails. A discussion is given in terms of configuration equivalents. It is concluded that the HPHF scheme seems to be useful for determining molecular properties specially in the case of large systems in which the more sophisticated methods are unmanageable.     

Solvation dynamics of electron produced by two-photon ionization of liquid polyols. II. Propanediols

Temporal evolution of transient absorption spectra of electrons produced by two-photon ionization of two isomers, propane-1,2-diol (12PD) and propane-1,3-diol (13PD), with 263 nm femtosecond laser pulses has been studied on picosecond time scale. The two-photon absorption coefficients of 12PD and 13PD at 263 nm were determined to be beta = (2.0 +/- 0.3) x 10(-11) and (2.4 +/- 0.3) x 10(-11) m W(-1), respectively. Time-resolved absorption spectra ranging from 440 to 720 nm have been measured, showing a blue shift for the first tens of picoseconds for both solvents. However, the observed solvation dynamics of electron appears faster in 13PD than in 12PD. The transient signals of electron solvation have then been reconstructed with different models (stepwise mechanism or continuous relaxation model) using a Bayesian data analysis method. Results are discussed, compared with those previously obtained in ethylene glycol (J. Phys. Chem. A 2006, 110, 1705) and corroborate the interpretation, according to which the solvation of electrons is mainly governed by continuous solvent molecular motions.     

Single-photon spectroscopy of singlet sulfur atoms and the autoionization lifetime measurements of the superexcited singlet states

Single-photon excitation spectra from the lowest singlet (1)D(2) level of sulfur atoms were recorded with a tunable vacuum ultraviolet (VUV) radiation source generated by frequency tripling in noble gases. The photolysis of CS(2) at 193 nm was used to produce the singlet S((1)D(2)) sulfur atoms that were then excited to neutral superexcited states with the tunable VUV radiation. These superexcited states undergo autoionization into the first ionization continuum state of S(+)((4)S(3/2) (o))+e(-), which is not directly accessible from the S((1)D(2)) state via an allowed transition. The excitation spectra were recorded by monitoring the S(+) signal in a velocity imaging apparatus while scanning the VUV excitation wavelength. Three new lines were observed in the spectra which have not been previously reported. The full widths at half maximum (FWHM) of each of the observed transitions were determined by fitting the profiles of each absorption resonances with the Fano formula. Autoionization lifetimes tau of these singlet superexcited states were obtained from FWHM using the Uncertainty Principle. Abnormal autoionization lifetimes were found for the 3s(2)3p(3)((2)D(o))nd((1)D(2)) and the 3s(2)3p(3)((2)D(o))ns((1)D(2)) Rydberg series, in which tau(5d) and tau(7s) are shorter than tau(4d) and tau(6s), respectively. This is contrary to the well-known scaling law of tau(n*) proportional, variantn(*3), which should be followed within a series unless there exist perturbations from other series or new channels open up to which some members of the series can decay. Possible perturbations from the nearby triplet series are suspected for causing the broadening of the 5d and 7s levels.