John W. Hepburn

Population transfer between two quantum states by piecewise chirping of femtosecond pulses: theory and experiment.

We propose and experimentally demonstrate the method of population transfer by piecewise adiabatic passage between two quantum states. Coherent excitation of a two-level system with a train of ultrashort laser pulses is shown to reproduce the effect of an adiabatic passage, conventionally achieved with a single frequency-chirped pulse. By properly adjusting the amplitudes and phases of the pulses in the excitation pulse train, we achieve complete and robust population transfer to the target state. The piecewise nature of the process suggests a possibility for the selective population transfer in complex quantum systems.

Threshold ion-pair production spectroscopy of HCl/DCl: Born–Oppenheimer breakdown in HCl and HCl+ and dynamics of photoion-pair formation

Threshold ion-pair production spectroscopy (TIPPS) has been applied to two isotopomers, HCl and DCl. From the high-resolution TIPP spectra the ion-pair thresholds of the two molecules have been precisely measured. Combined with the known ionization energy of H(D) and the electron affinity of Cl, the difference between their bond dissociation energies is calculated, and therefore an experimental determination of the effect of Born–Oppenheimer breakdown on the dissociation limit of the ground state potential curve has been obtained. The difference in De for the two isotopomers was found to be: De(H–Cl)−De(D–Cl)=3.2±1.0 cm−1. The bond energy for HCl was in agreement with our previous determination, D0(H–Cl)=35748.2±0.8 cm−1. These results are compared to a recent study of Born–Oppenheimer breakdown in HCl by Coxon and Hajigeorgiou, where high resolution spectroscopic data was used to fit Born–Oppenheimer breakdown correction terms for the intramolecular potential function. The present study also measured the...

Resonance enhanced multiphoton dissociation of polycyclic aromatic hydrocarbons cations in an RF ion trap

Abstract We present a novel method for recording the absorption spectra of cations of non-volatile molecules using resonance enhanced multiphoton dissociation of trapped ions. Ions are produced through a 2-laser desorption/ionization process in an RF ion trap mass-spectrometer. A third tunable visible laser induces fragmentation of the trapped ions with a yield dependent on the absorption cross-sections of the ions. Using this method we have recorded the visible absorption spectrum of two PAH isomers, phenanthrene and anthracene.

Effects of ultrafast molecular rotation on collisional decoherence.

Using an optical centrifuge to control molecular rotation in an extremely broad range of angular momenta, we study coherent rotational dynamics of nitrogen molecules in the presence of collisions. We cover the range of rotational quantum numbers between J=8 and J=66 at room temperature and study a crossover between the adiabatic and nonadiabatic regimes of rotational relaxation, which cannot be easily accessed by thermal means. We demonstrate that the rate of rotational decoherence changes by more than an order of magnitude in this range of J values and show that its dependence on J can be described by a simplified scaling law.

Complete characterization of molecular vibration using frequency resolved gating

The authors propose a new approach to vibration spectroscopy based on the coherent anti-Stokes Raman scattering of broadband ultrashort laser pulses. The proposed method reveals both the amplitude and the phase of molecular vibrations by utilizing the cross-correlation frequency resolved optical gating (XFROG) technique. The spectrum of the anti-Stokes pulse is measured as a function of the time delay between the laser-induced molecular vibrations and a well characterized broadband femtosecond probe pulse. The iterative XFROG algorithm provides a simultaneous complete characterization of molecular vibrations both in frequency and time domains with high resolution. They demonstrate experimentally the feasibility of the proposed method and show one of its potential applications in disentangling the time behavior of a mixture of vibrationally excited molecules. The technique of femtosecond pulse shaping is used for further improvement of accuracy and stability against noise.

A study of oleic acid and 2,4-DHB acid aerosols using an IR-VUV-ITMS: insights into the strengths and weaknesses of the technique

An investigation of oleic acid and 2,4-dihydroxybenzoic (DHB) acid aerosols was carried out using an aerosol mass spectrometer with pulsed lasers for vaporization and ionization and an ion trap for mass analysis. The extent of ion fragmentation was studied as a function of both vaporization energy and ionization wavelength. Low CO2 laser energies in the vaporization stage and near-threshold single photon ionization resulted in the least fragmented mass spectra. For DHB, only the molecular ion was observed, but for oleic acid fragmentation could not be eliminated. Tandem MS of the main fragment peak from oleic acid was carried out and provided a tool for compound identification. Photoionization efficiency curves were also collected for both DHB and oleic acid and the appearance energies of both parent and fragment ions were measured. Evidence for fragmentation occurring post-ionization is given by the similar appearance energies for both the parent and fragment ions. The results from this study were compared with those from similar experiments undertaken with time-of-flight (TOF) mass analyzers. The degree of fragmentation in the ion trap was considerably higher than that seen with TOF systems, particularly for oleic acid. This was attributed to the long storage interval in the ion trap which allows time for metastable ions to decay. Differences in the degree of fragmentation between the ion trap and TOF studies also provided further evidence for fragmentation occurring post-ionization. For 2,4-dihydroxybenzoic acid, the long delay prior to mass analysis also allowed time for reactions with background gases, in this case water, to occur.

Threshold ion-pair production spectroscopy of HCN.

The spectroscopic technique of threshold ion-pair production spectroscopy (TIPPS) has been applied to the triatomic molecule HCN. We have recorded the total ion-pair yield and TIPP spectra for the HCN-->H(+) + CN(-) process using coherent vacuum ultraviolet excitation. From the simulation of our high-resolution TIPP spectrum we have precisely measured the HCN ion-pair threshold E(IP) (0) to be 122 244 +/- 4 cm(-1). This value could be used to determine the bond dissociation energy D(0)(H-CN) to unprecedented accuracy. Our fitting result also showed that rotationally excited instead of cold CN(-) fragment is favored as the ion-pair dissociation product in the threshold region.

Narrowband spectroscopy by an all-optical correlation of broadband laser pulses

High-peak-power ultrafast lasers are widely used in nonlinear spectroscopy, but often limit its spectral resolution because of the broad frequency bandwidth of ultrashort laser pulses. Improving the resolution by achieving spectrally narrow excitation of, or emission from, the resonant medium by means of multiphoton interferences has been the focus of many recent developments in ultrafast spectroscopy. We demonstrate an alternative approach in which high resolution is exercised by detecting narrow spectral correlations between broadband excitation and emission optical fields. All-optical correlation analysis, easily incorporated into a traditional spectroscopic setup, enables direct, robust, and simultaneous detection of multiple narrow resonances in a single measurement.

Energetics and dynamics of threshold photoion-pair formation in HF∕DF

Threshold ion-pair production spectroscopy (TIPPS) has been applied to two isotopomers, HF and DF. From the high resolution (approximately 0.3 cm(-1)) TIPP spectra, the ion-pair thresholds of HFDF have been precisely measured. Combined with the ionization energy of H(D), the electron affinity of F, and the zero point energies of HFDF, the difference between their classical bond dissociation energies was obtained as D(e)(H-F)-D(e)(D-F) = 12.4 +/- 0.5 cm(-1). Our result provides an experimental estimate of the Born-Oppenheimer breakdown in the ground electronic state. The present work also measured the total ion-pair yield spectra of HF and DF in the threshold region, and the ion-pair formation mechanisms of these two molecules were discussed in light of the high resolution results.

Product correlations in photofragment dynamics

Correlations between either scalar or vector quantities measured in the study of photodissociation dynamics can serve to provide a very detailed picture of the dissociative event. This article discusses the use of Doppler profile and time-of-flight spectroscopy to learn about the correlation between the separate internal energies of two recoiling fragments, to study the way in which the internal energy distribution of a fragment varies with its recoil direction and to determine the angle between a photofragments recoil velocity direction and its rotation vector. Two new techniques are introduced. High-voltage switching of the potential applied to a time-of-flight mass spectrometer is used to map the velocity distribution of photofragments onto their arrival time distribution. Probing of photofragments by polarized light with sub-Doppler resolution is used to determine the degree of angular correlation between their rotation vector and their recoil velocity vector.