Two-photon Photoionization Research Articles

Hyperfine splitting, isotope shift, and level energy of the 3S states of (6,7)Li.

We study the 2S-3S transition of (6,7)Li by high-precision laser spectroscopy using two-photon Doppler-free excitation and photoionization detection. Interferometric cross referencing to metrologic Rb 3S-5D two-photon transitions allowed measurement of the transition isotope shift and hyperfine splitting in the 3S state with precision at the 30 kHz level. The results are IS=11 453.734(30) MHz, A(3S)(6Li)=35.263(15) MHz, and A(3S)(7Li)=93.106(11) MHz. Combined with recent theoretical work, the isotope shift yields a new value for the change in squared nuclear charge radii DeltaR(2)=0.47(5) fm(2). This is compared with other work and some existing discrepancies are resolved.

Evidence of temperature dependent activation barriers for near-threshold aqueous photoionization of 2′-deoxyguanosine and tryptophan

Alkaline 2′-deoxyguanosine undergoes one- and two-photon laser photoionization at 266 nm. Similar to tryptophan and indole, the one-photon quantum yield increases five-fold between 296 and 362 K. Earlier Arrhenius analyses of tryptophan and indole photoionization rate constants, yielded unreasonably large pre-exponential factors (1013–1018s−1). An analysis employing pre-exponential factors for diffusion controlled reactions (1010–1011s−1) provides evidence of activation barriers that decrease with increasing temperature over a range where water structure undergoes significant changes. In the context of Marcus theory, this analysis suggests that curvatures of the electron-transfer free energy surfaces decrease as temperature increases.

Two-Photon Photochemical Reactions of Aromatic Compounds in the Gas Phase

Studies on the gas-phase two-photon photochemistry of aromatic compounds performed in the past 25 years were summarized. Photophysical and photochemical unimolecular and bimolecular reactions occurring in the gas phase on the exposure of molecular systems (aliphatic aldehydes, ketones, and alcohols; aromatic hydrocarbons and their halogen derivatives; and heteroaromatic compounds with a conjugated π-electron system) to pulsed laser UV radiation were considered. Data on the kinetics and mechanisms of primary two-photon photoexcitation, photodissociation, and photoionization of aromatic molecules were systematized, and the nature of electronically excited states responsible for these processes was determined. Data on the reactivity of transient species and on the kinetics of elementary steps of laser UV photolysis (bimolecular charge transfer and heavy particle transfer reactions and termolecular association and cluster ion formation reactions) were presented. General prerequisites to the stability of aromatic compounds under non-steady-state conditions of UV photolysis were found in accordance with the electronic structure of reactants and with the donor–acceptor properties of mixture components.

Experimental two-dimensional torsional potential function for the methyl internal rotors in acetone

The two-dimensional torsional potential function for the methyl internal rotors in acetone, CH3COCH3, was derived by a global least-squares fit to spectroscopic data only. The data consisted of the torsional transitions in the far infrared and two-photon photoionization spectra and the torsionally split rotational transitions. This new potential function avoids previous contradictions in the values for the effective barrier to internal rotation. It contains potential top–top coupling terms as large as 66% of the effective barrier of 251±3cm−1. It is similar to the potential function predicted in recent ab initio calculations. The combination of data from rotational and vibrational spectroscopy was crucial to the achievement of a potential function that is compatible with both kinds of data.

The excited electronic states and the ionization potential of the AlND3 complex

The AlND3 complex was studied in a molecular beam by resonant one-color two-photon and two-color two-photon photoionization techniques with a resolution of 0.3 cm−1. Six progressions of bands were observed in the 18 100–26 200 cm−1 (381–552 nm) region. One progression (ωe=316±0.8 cm−1) was assigned to the Al–ND3 stretch (ν3′) in the B̃2A1 state correlating with the 3s 2S term of Al. The origin of the B̃2A1–X̃2E1/2 system was found at 18 532.5±0.7 cm−1. The second (ωe=428±2 cm−1) and third (ωe=594±12 cm−1) progressions were assigned to the Al–ND3 stretch (ν3′) and the bend (ν6′) in the C̃2E state correlating with the 3d 2D term of Al. The origin of the C̃2E–X̃2E1/2 system was found at 21 185±5 cm−1. Assignments of the other three progressions with origins and harmonic wave numbers of 22 667±2 and 358±2 cm−1; 24 382±10 and 432±10 cm−1; and ∼21 871 and ∼1050 cm−1 remain uncertain. The spin-orbit splitting of the X̃2E ground state was determined to be 55.8±0.7 cm−1. Several Rydberg series converging to the v3+=1, 2, and 3 levels of the ground state of the AlND3+ ion were observed. The ionization potential of the AlND3 complex was measured to be 39 710±2 cm−1. The ν3+ fundamental in the ground state of the AlND3+ ion was found to be 325±5 cm−1. To aid the interpretation of our spectra, ab initio optimized structures and vibrational wave numbers for the ground states of various isotopomers of AlNH3 and AlNH3+ were calculated. Also, Al–NH3 potential curves for several low-lying excited states were calculated. In addition, the ionization potential of the AlNH3 isotopomer was measured by the one-photon photoionization efficiency technique. The value of 39 760±10 cm−1 is an improvement on that previously reported by Di Palma et al. [Chem. Phys. Lett. 284, 184 (1998)], which was 180 cm−1 smaller.

Acid–base equilibria in aqueous solutions of 2-aminopurine radical cations generated by two-photon photoionization

Download options Please wait... Article information DOI https://doi.org/10.1039/A907995E Article type Paper Submitted 05 Oct 1999 Accepted 24 Nov 1999 First published 28 Jan 2000 Download Citation J. Chem. Soc., Perkin Trans. 2, 2000, 271-275 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Acid–base equilibria in aqueous solutions of 2-aminopurine radical cations generated by two-photon photoionization V. Shafirovich, A. Dourandin, N. P. Luneva and N. E. Geacintov, J. Chem. Soc., Perkin Trans. 2, 2000, 271 DOI: 10.1039/A907995E To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Vladimir Shafirovich Alexander Dourandin Natalia P. Luneva Nicholas E. Geacintov

Oxidation of Guanine at a Distance in Oligonucleotides Induced by Two-Photon Photoionization of 2-Aminopurine

Photoionization of free 2-aminopurine (2AP) in aqueous solutions, or of 2AP residues in oligonucleotides, is observed upon excitation with 308 nm XeCl excimer laser pulses (fwhm = 12 ns, ≤100 mJ/cm2/pulse) and involves the consecutive absorption of two photons. The absorption of light by the normal DNA bases at this wavelength is negligible under the same conditions. The kinetics and transient absorption spectra of the hydrated electrons and of 2AP radicals resulting from the deprotonation of the 2AP radical cations formed initially, have been investigated using standard spectroscopic methods. The 2AP radicals in aqueous solutions reversibly oxidize 2‘-deoxyguanosine and guanosine 5‘-monophosphate but do not react with the other three DNA nucleosides or nucleotides. The efficiency of photoionization of 2AP decreases in the following order: free 2AP > 2AP incorporated into single-stranded oligonucleotide > 2AP in double-stranded oligonucleotides. Photoionization of single 2AP residues incorporated into 18-mer single- and double-stranded oligonucleotides results in the oxidation of guanine residues in GG doublets. Employing gel electrophoresis methods, strand cleavage at these GG sites and at the site of the 2AP residue is observed after treatment of the irradiated oligonucleotides with hot piperidine. Using nanosecond transient absorption techniques, it is shown that the unimolecular oxidation of guanine by 2AP radicals at a distance can also be monitored directly in single-stranded oligonucleotides containing GG-doublets. It is found that increasing the number of nucleic acid bases between the 2AP radicals and the GG-doublet from 0 to 5 results in a decrease in the rate constant of guanine radical formation by a factor greater than 103.

Femtosecond Multiphoton Ionization Photoelectron Spectroscopy of the S2 State of Phenol

We investigated the short-lived S2 (1A1) state of phenol using two-photon resonance-enhanced photoionization coupled with photoelectron spectroscopy. Spectra were obtained by exciting the S2 state with femtosecond laser pulses at various wavelengths near 207 nm and ionizing with pulses at 207 or 414 nm. The spectra feature a strong transition to the ground electronic state of the ion (X) and a very weak transition to the first excited electronic state of the ion (A). In comparison, ionization via the longer-lived S1 state using 275 nm pulses generates slightly more A ions. We interpret the relative intensities of the A state signals as a reflection of configuration interactions in the intermediate S2 and S1 states. Time-delayed ionization experiments, as well as experiments with different laser pulse durations, suggest that the S2 state has a lifetime between 150 and 350 fs. Ionization via S1 produces a vibrationally resolved photoelectron spectrum, while ionization via S2 leads to a highly congested spe...

Photoionization spectral hole-burning and inversion symmetry of sites in doped insulators

The linear Stark shift of optical frequencies in impurity centers lacking inversion site symmetry results in various external electric field effects observed in persistent spectral hole-burning (PSHB) phenomena. This paper reviews some new effects of inversionless symmetry of centers in internal electric fields observed in PSHB of doped insulators with photoionization hole burning. (1) In multisite R-line ( 4A 2– 2E ) spectra of Cr 3+ ions, substituting Ge 4+ in the inversionless position in the crystal lattice of Li 2Ge 7O 15 (LGO), the two-photon self-gated photoionization PSHB was observed in the R-spectra of one site only. This drastic site dependence of PSHB demonstrates extremely strong (∼eV) relative shifts of the position within the host energy gap of the 4A 2, 2E levels of Cr 3+ ions, located at complex centers. These centers differ in the nature and the position of the nearby charge-compensating defect, where the Coulomb field produces different large linear Stark shifts of the electronic states of Cr 3+. (2) Photon-gated PSHB in f-d spectra of Eu 2+ multisites in CaS crystal was observed with unusually broad hole wings in the case of lines belonging to perturbed inversionless sites. This efficient off-resonant PSHB is due to strong linear Stark shifts of the f–d transition frequencies of Eu 2+ ions in time-varying Coulomb fields resulting from photoionization and recharging of Eu 2+–Eu 3+ ions during the hole-burning process in optically excited crystals.

The relaxation from linear to triangular Ag3 probed by femtosecond resonant two-photon ionization

We present extended NeNePo (negative to neutral to positive) measurements on the ultrafast dynamics in the ground state of neutral, mass-selected Ag3 molecules. A vibrational wave packet in the neutral molecule is created with an ultrashort laser pulse by photodetachment of the excess electron from the corresponding mass-selected anion. The subsequent molecular rearrangement is probed by photoionization after a selected time delay. Complementary to our previous investigations of this process, we now use two-photon photoionization via a resonant state in the probe step. Here, a bound–bound excitation to a well-known state followed by one-photon ionization is used instead of the nonresonant bound/free transition into the ionic continuum. Using radiation with wavelengths near 370 nm for resonant ionization, we observe a fast bending motion of the initially linear Ag3, followed by an ultrafast intramolecular vibrational energy redistribution, interpretable as an intramolecular collision process. The signal shows an apparent loss of vibrational coherence after the collision, which can be explained by the finite temperature of the anionic clusters in our experiment. Additionally, we describe a previously unknown resonance in the linear neutral molecule around 500 nm.

Photoionization spectroscopy of Ga-rare gas complexes

New electronic states, F 2Δ3/2, G 2Δ5/2, H 2Π1/2, and I 2Π3/2, are investigated for the complexes GaAr, GaKr, and GaXe with resonant two-photon photoionization spectroscopy. These excited states correlate to the 2D←2P (4d←4p) atomic transition of gallium. Vibronic structure in these spectra are used to obtain vibrational constants, and extrapolated progressions are used to determine dissociation energies. The upper 2Δ states are more than twice as strongly bound as the corresponding 2Π states. Excited state values of dissociation energies are used in energetic cycles to determine ground-state dissociation energies for GaAr, GaKr, and GaXe. In all three cases, the values obtained are significantly lower than previous estimates. The ground state of GaAr is extremely weakly bound, with D0″=20±20 cm−1, while the corresponding value for GaKr is only 35±20 cm−1. The B 2Σ+ excited states of the Ga–RG complexes are confirmed to have substantial barriers in their long-range potentials.

Spectroscopic Investigation of Photoinduced Charge Separation and Recombination in Solid Polymers

Charge generation in polymer films following photoexcitation of charge-transfer complexes and UV two-photon ionization of aromatic dopants was studied by transient absorption spectroscopy. Charge separation from photoinduced contact ion pairs is due to the hole migration away from the geminate radical anions during the lifetime of the exciplex, which occurs by a hopping mechanism. Hole trapping at dimeric sites and subsequent charge recombination give rise to delayed exterplex emission. In resonant two-photon photoionization experiments, radical cations of aromatic molecules and excess electrons were produced as the primary charged species. The thermalization length of electrons ejected from perylene by 337-nm photons was measured as 36 Å in solid polystyrene. Within the first 1 ns, reactions of electrons with polymer matrixes compete with the geminate electron−cation recombination, which leads to electron trapping and charge separation, with trapped ions stable up to milliseconds of time. Among the polymers studied, polystyrene shows the lowest reactivity and therefore the lowest yield of charge separation, ψPS = 2.4%, whereas poly(vinylbenzyl chloride) scavenges nearly all the excess electrons and exhibits the highest charge separation yield, ψPVBC = 1 at 210 K. Subsequent recombination of charge carriers trapped at different depths in solid polymer matrixes is diffusion limited over a wide dynamic range. The ion neutralization kinetics gradually evolves from geminate in nature to a homogeneous second-order reaction. The diffusivities of charge carriers were measured as 7.5 × 10-10 cm2/s for Cl- in poly(vinylbenzyl chloride), 2.2 × 10-10 cm2/s for CO2-• in poly(benzyl methacrylate), and 7.0 × 10-12 cm2/s for CO2-• in poly(methyl methacrylate).

Spectroscopic characterization of the metastable 3pπ 3Π+,0− valence states and the 4s3Σ+ Rydberg states of the MgKr and MgXe van der Waals molecules

The first metastable valence excited states and the first Rydberg states of the MgKr and MgXe molecules have been characterized by resonance two-photon photoionization (R2PI) spectroscopy. The Mg(3s3p 3PJ)⋅RG(3Π0+,0−) metastable states, produced by expanding the products of a laser-ablated magnesium rod in Kr/Ar or Xe/Ar gas mixtures into a supersonic expansion, were excited by a dye laser pulse to several vibrational levels of the Mg(3s4s 3S1)⋅RG(3Σ+) Rydberg states, with detection by ionization with a second dye laser pulse. Spectroscopic constants, bond energies, and bond lengths are reported for both states of MgKr and MgXe. The Σ+3 Rydberg states are much more strongly bound than the lower Π0−3 valence states, and in fact are essentially as strongly bound as the ground states of the analogous MgRG+ ions, characterized previously in the same apparatus. This clearly indicates that the RG atoms can readily penetrate the diffuse Mg(4s) Rydberg electron cloud. The interesting and unusual spin–orbit and “spin–spin” effects observed are attributed to mixing of some RG character into wave functions of predominantly Mg* excited state character. Bonding and spin–orbit interactions in the MgAr, MgKr, and MgXe first triplet metastable and Rydberg states are discussed.

Two-photon dissociative photoionization and photodissociation of single rotational states

We report on an experimental study of dissociative photoionization and neutral dissociation of by two-photon excitation from single rotational states (photon energy ). The B-state rovibrational levels are excited in the same light pulse by three-photon absorption from . Angle resolved photoelectron and ion kinetic energy distributions allow us to discriminate between the different decay channels. Neutral decay is detected by one-photon ionization of excited atomic hydrogen product states. The kinetic energy distribution of photoelectrons from dissociative photoionization shows an oscillatory structure which depends on the initial B-state rotational quantum number J. An indication of a corresponding structure in the ion kinetic energy distributions is only found for J = 0. Similar to results reported by Verschuur et al we find neutral decay preferentially into high principal quantum number atomic states. The results indicate that the doubly excited states play a crucial role in the decay processes.

Electron Transfer from Cyclohexane to Photoexcited Aromatic Ions. Generation and Kinetics of High-Mobility Solvent Holes

In cyclohexane solutions, aromatic radical cations are produced by two-photon (248 nm) photoionization of aromatic molecules. A novel excited state chemistry is observed; i.e., excitation of these radical cations with a 532 nm laser pulse, delayed by several microseconds, produces the high mobility cation of cyclohexane. The difference of the signals with and without the 532 nm pulse shows the decay of the cyclohexane radical cation free of complications from geminate ion recombination. The biexponential nature of this decay suggests an equilibrium involving the high mobility cation, such as was proposed to explain previous experimental results.

One- and two-photon spectroscopy of the 3s←n Rydberg transition of propionaldehyde (propanal)

Vacuum-UV (VUV) absorption and two-photon laser photoionization spectra of propionaldehyde vapour have been recorded at ambient temperature in the region of the 3s←n Rydberg transition and have been compared with resonance-enhanced multiphoton ionization (REMPI) spectra of the jet-cooled species. The vibrational analysis of the spectra has confirmed that the (n,3s) Rydberg state has cis and gauche conformational minima and that the electronic transition has two separate origins. An energy difference of 1260 cm −1 has been determined between the excited-state conformers, and an increase to 1290 cm −1 of the three-fold barrier to internal rotation of the methyl group on excitation of the (n,3s) state has been deduced. The unusual temperature dependence of the spectra has been explained by transitions from higher thermally populated levels of the aldehyde torsional vibration of the gauche conformers.

Strong bonding in a doubly excited valence state of a van der Waals molecule

The results of ab initio calculations, using extended Gaussian basis sets and including correlation energy through a variety of means, provide strong support for the Mg(3 pπ,3 pπ 3 P J) · Ar( 3Σ −) electronic-state assignment made by Massick and Breckenridge to spectral features in the resonant two-photon photoionization spectrum of vibrationally cold Mg(3 s,3 pπ 3 P J) · Ar( 3Π) van der Waals complexes. The doubly excited Mg(3 pπ,3 pπ 3 P J) · Ar( 3Σ −) state is found to be much more strongly bound than the singly excited Mg(3 s,3 pπ 3 P J) · Ar( 3Π) state, which, in turn, is much more bound than the Mg(3 s 2 1 S O ) · Ar( 3Σ +) ground state.

Complex-basis-function Floquet calculations of multiphoton ionization rates in hydrogen systems

We use the complex-basis-function technique to calculate one- and two-photon photoionization rates in intense light sources for atomic hydrogen, the hydrogen negative ion and the hydrogen molecular ion. For H2+ we also calculate three-photon ionization rates. Rather than calculating the ionization rates in the perturbation approximation, we take account of strong field effects by calculating complex quasienergies using a mixture of real and complex-basis functions. This allows us to extend the calculations from the perturbative to the non-perturbative regime as the laser intensity is increased. The method works well and gives reliable estimates of these rates.

Photoionization spectroscopy of LiAl

An electronic spectrum is observed for the new heteronuclear metal dimer, LiAl. This dimer is produced by laser vaporization of an alloy metal sample in a pulsed nozzle cluster source. Resonant two-photon photoionization (R2PI) is employed to collect this spectrum. We observe and characterize two excited electronic states for both the 6Li-Al and 7Li-Al isotopes. The analysis of hotbands provides the vibrational frequency (318.3cm −1) and dissociation energy (0.75eV) for the LiAl ground state, which both agree well with the predictions of ab initio theory.

Laser two-photon ionization spectrometry and photoionization threshold of perylene on the surface of water

There are a few bands in the two-photon ionization spectrum (wavelength dependence of the photoionization charge) of perylene on the water surface in the 340–440 nm region. The photoionization threshold of perylene on water was 5.95 ± 0.10 eV. The polarization energy of the positive ion ( P +) was determined as −0.95 eV, which was almost identical with that in trimethylpentane and was about half as much as the calculated value. The effective dielectric constant of water should be much smaller for surface photoionization.