Rotational Cooling Effect on the Rate Constant in the CHF + Ca Reaction at Low Collision Energies.

The rotational cooling effect on the reaction rate constant of the gas-phase ion-polar-molecule reaction CHF + Ca → CH + CaF was experimentally studied at low collision energies. Fluoromethane molecules showed higher reactivity as the rotational temperature decreased. The experimental rate constants were compared with the capture rate constants which were obtained by the Perturbed Rotational State (PRS) theory assuming the rotational level distribution corresponding to the experimental conditions.

Adaptively Optimized Gas Analysis Model with Deep Learning for Near-Infrared Methane Sensors.

Noise significantly limits the accuracy and stability of retrieving gas concentration with the traditional direct absorption spectroscopy (DAS). Here, we developed an adaptively optimized gas analysis model (AOGAM) composed of a neural sequence filter (NSF) and a neural concentration retriever (NCR) based on deep learning algorithms for extraction of methane absorption information from the noisy transmission spectra and obtaining the corresponding concentrations from the denoised spectra. The model was trained on two data sets, including a computationally generated one and the experimental one.

A study of the translational temperature dependence of the reaction rate constant between CHCN and Ne at low temperatures.

We have measured the translational temperature dependence of the reaction rate constant for CHCN + Ne → products at low temperatures. A cold Ne ensemble was embedded in Ca Coulomb crystals by a sympathetic laser cooling technique, while cold acetonitrile (CHCN) molecules were produced by two types of Stark velocity filters to widely change the translational temperatures. The measured reaction rate constant gradually increases with the decrease in the translational temperature of the velocity-selected CHCN molecules from 60 K down to 2 K, and thereby, a steep increase was observed at temperatures lower than 5 K.

Probing methane in air with a midinfrared frequency comb source.

We employed a midinfrared frequency comb source for methane detection in ambient air. The transmitted spectra over a bandwidth of about 500 nm were recorded with an optical spectrum analyzer under various experimental conditions of different path lengths. The normalized absorption spectra were compared and fitted with simulations, yielding quantitative values of concentrations of methane and water vapor in the ambient air.

Development of a wavy Stark velocity filter for studying interstellar chemistry.

Cold polar molecules are key to both the understanding of fundamental physics and the characterization of the chemical evolution of interstellar clouds. To facilitate such studies over a wide range of temperatures, we developed a new type of Stark velocity filter for changing the translational and rotational temperatures of velocity-selected polar molecules without changing the output beam position. The translational temperature of guided polar molecules can be significantly varied by exchanging the wavy deflection section with one having a different radius of the curvature and a different deflection angle.

Single ion fluorescence excited with a single mode of an UV frequency comb.

Optical frequency combs have revolutionized the measurement of optical frequencies and improved the precision of spectroscopic experiments. Besides their importance as a frequency-measuring ruler, the frequency combs themselves can excite target transitions (direct frequency comb spectroscopy). The direct frequency comb spectroscopy may extend the optical frequency metrology into spectral regions unreachable by continuous wave lasers.

Near infrared frequency comb vernier spectrometer for broadband trace gas detection.

We present a femtosecond frequency comb vernier spectrometer in the near infrared with a femtosecond Er doped fiber laser, a scanning high-finesse cavity and an InGaAs camera. By utilizing the properties of a frequency comb and a scanning high-finesse cavity such a spectrometer provides broad spectral bandwidth, high spectral resolution, and high detection sensitivity on a short time scale. We achieved an absorption sensitivity of ~8 × 10(-8) cm(-1)Hz(-1/2), corresponding to a detection limit of ~70 ppbv for acetylene, with a resolution of ~1.

Generation of femtosecond optical vortices by molecular modulation in a Raman-active crystal.

We have generated multi-color optical vortices in a Raman-active crystal PbWO4 using two-color Fourier-transform limited femtosecond laser pulses. This setup overcomes some of the limitation of our previous research by allowing for the production of subcycle femtosecond optical vortices without the need for compensating for added chirp. In addition, the use of an OPA allows for greater flexibility in exciting different Raman modes.

Femtosecond electron-lattice thermalization dynamics in a gold film probed by pulsed surface plasmon resonance.

The dynamics of electronic excitations and their relaxation in a gold film is studied on the femtosecond time scale with a pump-probe technique. For the pump beam we use pulses with wavelengths centered at 800 nm, 400 nm or both. The surface plasmon resonance (SPR) in Kretschmann's configuration is used as a sensitive and fast-response probe of the dynamics of the dielectric properties of the gold film.

High-power mid-infrared frequency comb source based on a femtosecond Er:fiber oscillator.

We report on a high-power mid-infrared (MIR) frequency comb source based on a femtosecond (fs) Er:fiber oscillator with a stabilized repetition rate of 250 MHz. The MIR frequency comb is produced through difference frequency generation in a periodically poled MgO-doped lithium niobate crystal. The output power is about 120 mW, with a pulse duration of about 80 fs and spectrum coverage from 2.

Krypton separation from ambient air for application in collinear fast beam laser spectroscopy.

A portable apparatus for the separation of krypton from environmental air samples was tested. The apparatus is based on the cryogenic trapping of gases at liquid nitrogen temperature followed by controlled releases at higher temperatures. The setup consists of a liquid nitrogen trap for the removal of H(2)O and CO(2), followed by charcoal-filled coils that sequentially collect and release krypton and other gases providing four stages of gas chromatography to achieve separation and purification of krypton from mainly N(2), O(2), and Ar.

Interaction and spectral gaps of surface plasmon modes in gold nano-structures.

The transmission of ultrashort (7 fs) broadband laser pulses through periodic gold nano-structures is studied. The distribution of the transmitted light intensity over wavelength and angle shows an efficient coupling of the incident p-polarized light to two counter-propagating surface plasmon (SP) modes. As a result of the mode interaction, the avoided crossing patterns exhibit energy and momentum gaps, which depend on the configuration of the nano-structure and the wavelength.

Spectral phase retrieval from interferometric autocorrelation by a combination of graduated optimization and genetic algorithms.

We describe a method for retrieving spectral phase information from second harmonic interferometric autocorrelation measurements supplemented by the use of the observed spectral intensity. By applying a combination of graduated optimization and genetic algorithms, accurate phase retrieval of laser pulses as short as a few optical cycles was obtained from the measured autocorrelation and spectral intensity. The effectiveness of the combined algorithms is demonstrated on a set of significantly different femtosecond pulse shapes.

Two-photon fluorescence of Coumarin 30 excited by optimally shaped pulses.

We optimized the two-photon fluorescence (TPF) of a Coumarin 30 dye by using a feedback-controlled femtosecond pulse shaping technique. For optimization we implemented an evolutionary algorithm with a liquid crystal phase-only pulse shaper in a folded 4f setup. The optimization procedure applied to the second harmonic generation, and TPF noticeably improved the output signals and demonstrated good convergence.

Propagation of ultrashort laser pulses in water: linear absorption and onset of nonlinear spectral transformation.

We study propagation of short laser pulses through water and use a spectral hole filling technique to essentially perform a sensitive balanced comparison of absorption coefficients for pulses of different duration. This study is motivated by an alleged violation of the Bouguer-Lambert-Beer law at low light intensities, where the pulse propagation is expected to be linear, and by a possible observation of femtosecond optical precursors in water. We find that at low intensities, absorption of laser light is determined solely by its spectrum and does not directly depend on the pulse duration, in agreement with our earlier work and in contradiction to some work of others.

Propagation length of surface plasmons in a metal film with roughness.

The propagation of laser-excited surface plasmons along a gold film with surface roughness is directly observed via scattered light. The attenuation length of surface plasmons in a broad wavelength interval is calculated for smooth gold and silver films. The surface roughness, which was characterized with an AFM, introduces corrections to the attenuation length, angular dependence of the surface plasmon resonance, and the effective dielectric constant of the metal film.

Impedance spectroscopy of alpha-beta tubulin heterodimer suspensions.

Impedance spectroscopy is a technique that reveals information, such as macromolecular charges and related properties about protein suspensions and other materials. Here we report on impedance measurements over the frequency range of 1 Hz to 1 MHz of alpha-beta tubulin heterodimers suspended in a buffer. These and other polyelectrolyte suspensions show enormous dielectric responses at low frequencies, due both to the motion of charges suspended in the medium and to an electrical double layer that forms at each electrode-medium interface.

Visible and UV coherent Raman spectroscopy of dipicolinic acid.

We use time-resolved coherent Raman spectroscopy to obtain molecule-specific signals from dipicolinic acid (DPA), which is a marker molecule for bacterial spores. We use femtosecond laser pulses in both visible and UV spectral regions and compare experimental results with theoretical predictions. By exciting vibrational coherence on more than one mode simultaneously, we observe a quantum beat signal that can be used to extract the parameters of molecular motion in DPA.

A frequency comb in the extreme ultraviolet.

Since 1998, the interaction of precision spectroscopy and ultrafast laser science has led to several notable accomplishments. Femtosecond laser optical frequency 'combs' (evenly spaced spectral lines) have revolutionized the measurement of optical frequencies and enabled optical atomic clocks. The same comb techniques have been used to control the waveform of ultrafast laser pulses, which permitted the generation of single attosecond pulses, and have been used in a recently demonstrated 'oscilloscope' for light waves.

Effect of varying electric potential on surface-plasmon resonance sensing.

The high sensitivity of surface-plasmon resonance (SPR) sensors allows measurements of small variations in surface potentials to be made. We studied the changes of the SPR angle when an oscillating electric potential was applied to a gold film on which surface plasmons were excited. The shifts of the SPR resonance angle were observed for various aqueous solutions as an adjacent medium.