Non-resonant Two-photon Ionization Research Articles

Valence Electronic Structure of Interfacial Phenol in Water Droplets.

Biochemistry and a large part of atmospheric chemistry occur in aqueous environments or at aqueous interfaces, where (photo)chemical reaction rates can be increased by up to several orders of magnitude. The key to understanding the chemistry and photoresponse of molecules in and "on" water lies in their valence electronic structure, with a sensitive probe being photoelectron spectroscopy. This work reports velocity-map photoelectron imaging of submicrometer-sized aqueous phenol droplets in the valence region after nonresonant (288 nm) and resonance-enhanced (274 nm) two-photon ionization with femtosecond ultraviolet light, complementing previous liquid microjet studies. For nonresonant photoionization, our concentration-dependent study reveals a systematic decrease in the vertical binding energy (VBE) of aqueous phenol from 8.0 ± 0.1 eV at low concentration (0.01 M) to 7.6 ± 0.1 eV at high concentration (0.8 M). We attribute this shift to a systematic lowering of the energy of the lowest cationic state with increasing concentration caused by the phenol dimer and aggregate formation at the droplet surface. Contrary to nonresonant photoionization, no significant concentration dependence of the VBE was observed for resonance-enhanced photoionization. We explain the concentration-independent VBE of ∼8.1 eV observed upon resonant ionization by ultrafast intermediate state relaxation and changes in the accessible Franck-Condon region as a consequence of the lowering of the intermediate state potential energy due to the formation of phenol excimers and excited phenol aggregates. Correcting for the influence of electron transport scattering in the droplets reduced the measured VBEs by 0.1-0.2 eV.

Ponderomotive shifts induced by intense laser-driven coherent extreme ultraviolet radiation

Signatures of strong-field effects induced by an intense attosecond pulse train, synthesized by the superposition of a Ti:sapphire laser high-order harmonics are observed in atomic two-photon ionization. Ponderomotive shifts in the photoelectrons’ kinetic energies have been measured in the non-resonant two-photon ionization of Helium atoms induced by the interaction with extreme ultraviolet radiation at intensities on the order of 1015 W cm−2. These results inaugurate the topic of strong-field phenomena in experimental laser based attosecond science.

Role of dipole-forbidden autoionizing resonances in nonresonant one-color two-photon single ionization of N2

We present an experimental and theoretical energy- and angle-resolved study on the photoionization dynamics of non-resonant one-color two-photon single valence ionization of neutral N$_2$ molecules. Using 9.3 eV photons produced via high harmonic generation and a 3-D momentum imaging spectrometer, we detect the photoelectrons and ions produced from one-color two-photon ionization in coincidence. Photoionization of N$_2$ populates the X $^2\Sigma^+_g$, A $^2\Pi_u$, and B $^2\Sigma^+_u$ ionic states of N$_2^+$, where the photoelectron angular distributions associated with the X $^2\Sigma^+_g$ and A $^2\Pi_u$ states both vary with changes in photoelectron kinetic energy of only a few hundred meV. We attribute the rapid evolution in the photoelectron angular distributions to the excitation and decay of dipole-forbidden autoionizing resonances that belong to series of different symmetries, all of which are members of the Hopfield series, and compete with the direct two-photon single ionization.

Angle-resolved nonresonant two-photon single ionization of argon using 9.3-eV photons produced via high-order harmonic generation

We present an experimental study on the photoionization dynamics of non-resonant one-color two-photon single valence ionization of neutral argon atoms. Using 9.3 eV photons produced via high harmonic generation and a 3-D momentum imaging spectrometer, we detect the photoelectrons and ions produced from non-resonant two-photon ionization in coincidence. Photoionization from the $3p$ orbital produces a photoelectron scattering wave function with $p$ and $f$ partial wave components, which interfere and result in a photoelectron angular distribution with peak amplitude perpendicular to the VUV polarization. The comparison between the present results and two previous sets of theoretical calculations [Pan, C. & Starace, A. F. (1991). $\textit{Physical Review A}$, 44(1), 324., and Moccia, R., Rahman, N. K., & Rizzo, A. (1983). $\textit{Journal of Physics B: Atomic and Molecular Physics}$, 16(15), 2737.] indicates that electron-electron correlation contributes appreciably to the two-photon ionization dynamics.

Analysis of absorption time delays in streaking of resonant and non-resonant two-photon ionization

We theoretically study the determination of the time delay between the subsequent absorption of photons in two-photon ionization with the attosecond streaking technique. An analytical model for the absorption time delay based on analysis of the process using perturbation theory is obtained. Model predictions at central frequencies that are resonant and non-resonant with an intermediate bound state agree well with results of numerical streaking simulations, if the intermediate state is well-isolated from other states and the effect of the streaking field on the intermediate state is negligible.

Photoelectron distribution of nonresonant two-photon ionization of neutral atoms

Photoelectron angular distributions following the non-resonant two-photon K-shell ionization of neutral atoms are studied theoretically. Using the independent particle approximation and relativistic second-order perturbation theory, the contributions of screening and relativistic effects to the photoelectron angular distribution are evaluated. A simple nonrelativistic expression is presented for the angle-differential cross section in dipole approximation for two-photon ionization by elliptically polarized photons, and its limitations are analyzed numerically. Moreover, we show that screening effects of the inactive electrons can significantly affect the photoelectron distributions and can also lead to a strong elliptical dichroism. Numerical results are resented for the case of two-photon K-shell ionization of neutral Ne, Ge, Xe, and U atoms.

Determination of Nerve Agent Metabolites by Ultraviolet Femtosecond Laser Ionization Mass Spectrometry.

Nerve agent metabolites, i.e., isopropyl methylphosphonic acid (IMPA) and pinacolyl methylphosphonic acid (PMPA), were derivatized by reacting them with 2,3,4,5,6-pentafluorobenzyl bromide (PFBBr) and were determined by mass spectrometry using an ultraviolet femtosecond laser emitting at 267 and 200 nm as the ionization source. The analytes of the derivatized compounds, i.e., IMPA-PFB and PMPA-PFB, contain a large side-chain, and molecular ions are very weak or absent in electron ionization mass spectrometry. The use of ultraviolet femtosecond laser ionization mass spectrometry, however, resulted in the formation of a molecular ion, even for compounds such as these that contain a highly bulky functional group. The signal intensity was larger at 200 nm due to resonance-enhanced two-photon ionization. In contrast, fragmentation was suppressed at 267 nm (nonresonant two-photon ionization) especially for PMPA-PFB, thus resulting in a lower background signal. This favorable result can be explained by the small excess energy in ionization at 267 nm and by the low-frequency vibrational mode of a bulky trimethylpropyl group in PMPA.

Relativistic effects in the non-resonant two-photon K-shell ionization of neutral atoms

Relativistic effects in the non-resonant two-photon K-shell ionization of neutral atoms are studied theoretically within the framework of second-order perturbation theory. The non-relativistic results are compared with the relativistic calculations in the dipole and no-pair approximations as well as with the complete relativistic approach. The calculations are performed in both velocity and length gauges. Our results show a significant decrease of the total cross section for heavy atoms as compared to the non-relativistic treatment, which is mainly due to the relativistic wavefunction contraction. The effects of higher multipoles and negative continuum energy states counteract the relativistic contraction contribution, but are generally much weaker. While the effects beyond the dipole approximation are equally important in both gauges, the inclusion of negative continuum energy states visibly contributes to the total cross section only in the velocity gauge.

Relativistic calculations of the nonresonant two-photon ionization of neutral atoms

The non-resonant two-photon one-electron ionization of neutral atoms is studied theoretically in the framework of relativistic second-order perturbation theory and independent particle approximation. In particular, the importance of relativistic and screening effects in the total two-photon ionization cross section is investigated. Detailed computations have been carried out for the K-shell ionization of neutral Ne, Ge, Xe, and U atoms. The relativistic effects significantly decrease the total cross section, for the case of U, for example, they reduce the total cross section by a factor of two. Moreover, we have found that the account for the screening effects of the remaining electrons leads to occurrence of an unexpected minimum in the total cross section at the total photon energies equal to the ionization threshold, for the case of Ne, for example, the cross section drops there by a factor of three.

Determination of Fragrance Allergens by Ultraviolet Femtosecond Laser Ionization Mass Spectrometry.

The allergenic compounds listed in the Cosmetics Directive by the Scientific Committee for Consumer Safety were analyzed by gas chromatography combined with multiphoton ionization mass spectrometry using a femtosecond laser emitting at 200 and 267 nm as the ionization source. The limits of detection were less than 100 pg/μL for all of the compounds, permitting them to be measured in actual samples that were simply prepared by a 100-fold dilution of the original sample. The ionization process was investigated for the 26 allergens, some of which had no absorption band, even in the far-UV region. As a result, nonresonant two-photon ionization was found to be the most sensitive and universal method for the trace analysis of these compounds, because of the short pulse width, i.e., a high peak power, of the femtosecond laser used. It should be noted that the excess energy can be reduced by using a laser emitting at longer wavelengths (267 nm) and that fragmentation can be suppressed, especially for a molecule that contains a long side chain. Three commercially available perfumes were measured, and more than 10 allergenic compounds were determined.

Nonresonant EUV-UV two-color two-photon ionization of He studied by single-shot photoelectron spectroscopy

Single-shot photoelectron spectroscopy of nonresonant two-color two-photon ionization of He has been carried out using an intense free-electron laser (FEL) at 59.7 nm and a femtosecond UV laser at 268 nm. By shot-by-shot analysis of photoelectron spectra, the cross section for nonresonant two-color ionization of He is determined to be ${\ensuremath{\sigma}}^{(2)}=4.1(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}52}$ cm${}^{4}$ s. The cross-correlation trace derived from the nonresonant signals shows that the temporal resolution of the system is 0.36(3) ps, which is mainly governed by the timing jitter between the FEL and UV laser pulses.

Autocorrelator for measuring an ultrashort optical pulse width in the ultraviolet region based on two-photon ionization of an organic compound

An autocorrelator consisting of a time-of-flight mass spectrometer as a two-photon-response detector was constructed for use in measuring an optical pulse width based on non-resonant two-photon ionization. An organic compound that can be two-photon ionized in a wide spectral region has the potential for use in measuring an optical pulse width approaching 1fs.

Searching for a molecule with a wide frequency domain for non-resonant two-photon ionization to measure the ultrashort optical pulse width

The frequency domain of non-resonant two-photon ionization, defined as a frequency separation from half of the ionization energy to the excitation energy, was investigated using density functional theory calculations. Several organic compounds have the potential for use in the measurement of the ultrashort optical pulse width in the ultraviolet region using an autocorrelator consisting of a mass spectrometer as a two-photon-response detector.

Determination of the absolute two-photon ionization cross section of He by an XUV free electron laser

The resonant and non-resonant two-photon single ionization processes of He were investigated using intense free electron laser light in the extreme ultraviolet (XUV) region (53.4–61.4 nm) covering the 1s–2p and 1s–3p resonant transitions of He. On the basis of the dependences of the yield of He+ on the XUV light-field intensity at 53.4, 58.4, 56.0 and 61.4 nm, the absolute values of the two-photon ionization cross sections of He at the four different wavelengths and their dependence on the light-field intensity were determined for the first time.

Phase and ellipticity dependence of the photoelectron angular distribution in non-resonant two-photon ionization of atomic hydrogen. I

We study the ellipticity and the dependence on the phase lag (lead) (between the semimajor and the semiminor axes of the field components) of the photoelectron angular distribution (PAD) in the non-resonant two-photon ionization of atomic hydrogen. We establish exact analytical expressions for azimuthal PAD for 3s, 3p and 3d excited initial states, marked by the occurrence of an asymmetric term. This term gives rise to elliptic dichroism (ED), which can be obtained in two ways: either with the left (versus right) ellipticity, or with the phase lag (versus lead); for 3s and 3p initial states, it is shown that the quantum phase of continua is directly related to the phase lag, one-photon below-threshold ionization, and indirectly one photon above. Another important result is that the magnetic sublevels, m = 0, for 3p and m = ±1, for 3d, do not contribute to the azimuthal PAD. Our numerical results show, for 3s and 3d, and near-threshold ionization, that the PAD has maxima either along the semimajor or the semiminor axis, while for above-threshold ionization, they are always shifted from these axes. However, the maxima of the corresponding ED coincide with the PAD maxima, while for 3p, they are shifted from the PAD minima. A strong dependence of the ED sign is noted, regardless of the state or the process. However, strong ED signals are obtained for the 3s initial state and below-threshold ionization.

Measurement of the isotope selectivity of the strontium magneto-optical trap by a time-of-flight mass spectrometer

The isotope selectivity of the strontium (Sr) magneto-optical trap (MOT) is measured by a time-of-flight (TOF) mass spectrometer. The thermally generated Sr atoms are slowed down by the Zeeman slowing technique and trapped by the magneto-optical method. The trapped atoms are photoionized by the nonresonant two-photon ionization, and the generated ions are analyzed by the TOF mass spectrometer. The isotope selectivity of the MOT is up to approximately 2000 for 86Sr, and the overall selectivity of the system, which consists of the MOT and the mass spectrometer is also discussed.

Relativistic and retardation effects in the two-photon ionization of hydrogen-likeions

The non-resonant two-photon ionization of hydrogen-like ions is studied insecond-order perturbation theory, based on the Dirac equation. To carry outthe summation over the complete Coulomb spectrum, a Green functionapproach has been applied to the computation of the ionization cross sections.Exact second-order relativistic cross sections are compared with data asobtained from a relativistic long-wavelength approximation as well asfrom the scaling of non-relativistic results. For high-Zions, the relativistic wavefunction contraction may lower the two-photonionization cross sections by a factor of two or more, while retardation effectsappear less pronounced but still give rise to non-negligible contributions.

Nonresonant ionization detected IR spectrum of jet-cooled phenol. Ionization mechanism and its application to overtone spectroscopy

Vibrational transitions of jet-cooled phenol have been detected by nonresonant two-photon ionization due to a UV laser from 3400 to 14000 cm −1. The UV frequency dependence of the IR-UV double resonance signals is used for discussion on the mechanism of ionization. The spectrum shows a well-resolved structure due to the first to the fourth quantum of OH stretching vibrations, CH overtones and various combination vibrations. The vibrational frequency, anharmonicity and the dissociation energy of the OH stretching mode has been measured. The bandwidth of the OH overtone is found to decrease with increase in the vibrational quantum number.

Calculation of the differential two-photon ionization cross section of H2S

Differential cross section of nonresonant two-photon ionization of the outer valence shell of H2S is computed using ab initio methods for several choices of photon polarization and experimental setup. The theoretical model employed is based on the second-order perturbation theory in the dipole and vertical transition approximations and the calculations are performed adopting a large basis set of L2 integrable functions. The excitations to intermediate states are described in the random phase approximation, while the final states are described in the static-exchange approximation adopting for the continuum orbitals a K-matrix technique that allows a proper description of the angular distribution of the emitted electrons.

The significance of autoionization processes during nonresonant one-color two-photon ionization of neutral silver and copper atoms at λ = 248 nm

Recent experimental ionization efficiency data (photoion signal as a function of laser intensity) for nonresonant two-photon ionization of Ag and Cu at λ = 248 nm show strong deviations from the expected laser intensity dependence of the nonresonant two-photon ionization process. In addition, the comparison of the ionization efficiency data of the photoions Ag + and Cu + shows drastic differences between silver and copper. These deviations are explained by considering a two-photon excitation process of the internal 4d (Ag) or 3d (Cu) electron followed by autoionization of the short-lived excited atomic states produced. In the case of Cu, the competition between the de-excitation due to autoionization processes and non-ionizing radiative de-excitation schemes must also be taken into account. The importance of these processes for quantitative surface analysis using secondary neutral mass spectrometry in combination with nonresonant two-photon postionization is addressed.