Tunable Laser Beam Research Articles

Vibrational Signatures of Protonated, Phosphorylated Amino Acids in the Gas Phase

Structural characterization of protonated phosphorylated serine, threonine, and tyrosine was performed using mid-infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations. The ions were generated and analyzed by an external electrospray source coupled to a Paul ion-trap type mass spectrometer. Their fragmentation was induced by the resonant absorption of multiple photons from a tunable free electron laser (FEL) beam. IRMPD spectra were recorded in the 900-1850 cm(-1) energy range and compared to the corresponding computed IR spectra. On the basis of the frequency and intensity of two independent bands in the 900-1400 cm(-1) energy range, it is possible to identify the phosphorylated residue. IRMPD spectra for a 12-residue fragment of stathmin in its phosphorylated and nonphosphorylated forms were also recorded in the 800-1400 cm(-1) energy range. The lack of spectral congestion in the 900-1300 cm(-1) region makes their distinction facile. Our results show that IRMPD spectroscopy may became a valuable tool for structural characterization of small phosphorylated peptides.

Towards optical spectroscopy of the element nobelium ( ${\sf Z }= \mathsf{102}$ ) in a buffer gas cell

For the investigation of the atomic level structure of heavy elements which can only be produced at on-line facilities such as GSI, a novel experimental procedure has been developed. It is based on Radiation Detected Resonance Ionization Spectroscopy (RADRIS) and can be applied to elements like nobelium produced at rates of a few ions per second. Fusion reaction products are separated from the primary beam by the velocity filter SHIP at GSI, stopped in a buffer gas cell, collected on a tantalum filament and then re-evaporated as atoms. The ions produced by resonance ionization with tunable laser beams are detected via their characteristic α decay. First on-line experiments on α-active 155 Yb, which is supposed to have an atomic level structure similar to nobelium, were performed. These test experiments focused on the optimization of the collection and re-evaporation process of the radioactive ions, the laser ionization efficiency and the detection via α decay. An overall efficiency for RADRIS of 0.8% with respect to the target production rate was measured. While further improvements of this efficiency are in progress it should already be sufficient for the search for atomic levels in nobelium.

248 nm photolysis of CH2Br2 by using cavity ring-down absorption spectroscopy: Br2 molecular elimination at room temperature

Following photodissociation of CH2Br2 at 248 nm, Br2 molecular elimination is detected by using a tunable laser beam, as crossed perpendicular to the photolyzing laser beam in a ring-down cell, probing the Br2 fragment in the B 3Piou+ -X 1Sigmag+ transition. The nascent vibrational population is obtained, yielding a population ratio of Br2(v = 1)Br2(v = 0) to be 0.7 +/- 0.2. The quantum yield for the Br2 elimination reaction is determined to be 0.2 +/- 0.1. Nevertheless, when CH2Br2 is prepared in a supersonic molecular beam under cold temperature, photofragmentation gives no Br2 detectable in a time-of-flight mass spectrometer. With the aid of ab initio potential energy calculations, a plausible pathway is proposed. Upon excitation to the 1B1 or 3B1 state, C-Br bond elongation may change the molecular symmetry of Cs and enhance the resultant 1 1,3A'-X 1A' (or 1 1,3B1-X 1A1 as C2v is used) coupling to facilitate the process of internal conversion, followed by asynchronous concerted photodissociation. Temperature dependence measurements lend support to the proposed pathway.

Development of improved PLIF CH detection using an Alexandrite laser for single-shot investigation of turbulent and lean flames

We report on the development of planar laser-induced fluorescence (PLIF) for CH imaging with improved detection sensitivity for single-shot investigations of turbulent, lean, premixed flames. A ring-cavity, pulsed Alexandrite laser was frequency-doubled to excite the lines in the R-branch band-head of the B–X (0,0) band and broadband fluorescence from the B–X (0,1), A–X (1,1) and (0,0) bands, overlapping in the spectral range around 431 nm, was collected. The employed Alexandrite laser, which is characterized by its long pulse duration (150 ns), gives a tunable laser beam around 775 nm with a pulse energy for the second harmonic at the CH absorption wavelength of about 70 mJ. Moreover, the laser has the possibility to be operated in narrow bandwidth (100 MHz) or broad bandwidth (8 cm −1). An introductory high resolution excitation scan over the R-branch band-head was performed and, in addition, saturated excitation with the broadband option of the laser was investigated. By simultaneous excitation of several rotational transitions and to bring these transitions close to saturation, high signal-to-noise ratios were reached over a wide range of equivalence ratios. A sharp and thin CH layer was observed in single-shot PLIF images from laminar premixed methane/air flames from Φ = 0.6 to Φ = 1.5. Finally, the impact of the developed CH PLIF technique is demonstrated in a highly turbulent, lean, partially premixed methane/air flame established on a co-axial jet flame burner.

Br 2 elimination in 248-nm photolysis of CF2Br2 probed by using cavity ring-down absorption spectroscopy

By using cavity ring-down absorption spectroscopy technique, we have observed the channel of Br2 molecular elimination following photodissociation of CF2Br2 at 248 nm. A tunable laser beam, which is crossed perpendicular to the photolyzing laser beam in a ring-down cell, is used to probe the Br2 fragment in the B 3Piou+-X1Sigmag+ transition. The vibrational population is obtained in a nascent state, despite ring-down time as long as 500-1000 ns. The population ratio of Br2(v=1)/Br2(v=0) is determined to be 0.4+/-0.2, slightly larger than the value of 0.22 evaluated by Boltzmann distribution at room temperature. The quantum yield of the Br2 elimination reaction is also measured to be 0.04+/-0.01. This work provides direct evidence to support molecular elimination occurring in the CF2Br2 photodissociation and proposes a plausible pathway with the aid of ab initio potential-energy calculations. CF2Br2 is excited probably to the 1B1 and 3B2 states at 248 nm. As the C-Br bond is elongated upon excitation, the coupling of the 1A'(1B1) state to the high vibrational levels of the ground state X 1A'(1A1) may be enhanced to facilitate the process of internal conversion. After transition, the highly vibrationally excited CF2Br2 feasibly surpasses a transition barrier prior to decomposition. According to the ab initio calculations, the transition state structure tends to correlate with the intermediate state CF2Br+Br(CF2Br...Br) and the products CF2+Br2. A sequential photodissociation pathway is thus favored. That is, a single C-Br bond breaks, and then the free-Br atom moves to form a Br-Br bond, followed by the Br2 elimination. The formed Br-Br bond distance in the transition state tends to approach equilibrium such that the Br2 fragment may be populated in cold vibrational distribution. Observation of a small vibrational population ratio of Br2(v=1)Br2(v=0) agrees with the proposed mechanism.

Molecular elimination of Br2 in 248 nm photolysis of bromoform probed by using cavity ring-down absorption spectroscopy.

By using cavity ring-down spectroscopy technique, we have observed the channel leading to Br(2) molecular elimination following photodissociation of bromoform at 248 nm. A tunable laser beam, which is crossed perpendicular to the photolysis laser beam in a ring-down cell, is used to probe the Br(2) fragment in the B(3)Pi(ou)(+)-X(1)Sigma(g)(+) transition using the range 515-524 nm. The ring-down time lasts 500 ns, so the rotational population of the Br(2) fragment may not be nascent nature, but its vibrational population should be. The vibrational population ratio of Br(2)(upsilon=1)/Br(2)(upsilon=0)=0.8+/-0.2 implies that the fragmented Br(2) is vibrationally hot. The quantum yield of the molecular elimination reaction is 0.23+/-0.05, consistent with the values of 0.26 and 0.16 reported in 234 and 267 nm photolysis of bromoform, respectively, using velocity ion imaging. A plausible photodissociation pathway is proposed, based upon this work and ab initio calculations. The A(1)A(2), B(1)E, and C(1)A(1) singlet states of bromoform are probably excited at 248 nm. These excited states may couple to the high vibrational levels of the ground state X(1)A(1) via internal conversion. This vibrationally excited bromoform readily surpasses a reaction barrier 389.6 kJ/mol prior to decomposition. The transition state structure tends to correlate with vibrationally hot Br(2). Dissociation after internal conversion of the excited states to vibrationally excited ground state should result in a large fraction of the available energy to be partitioned in vibrational states of the fragments. The observed vibrationally hot Br(2) fragment seems to favor the dissociation pathway from high vibrational levels of the ground state. Nevertheless, the other reaction channel leading to a direct impulsive dissociation from the excited states cannot be excluded.

Microscopy of localized second-harmonic enhancement in random metal nanostructures

Using second-harmonic (SH) scanning optical microscopy in reflection we study the effect of localized SH enhancement in random metal nanostructures. With a tightly focused tunable (750--830 nm) laser beam, we obtain SH images of a 55-nm-thick gold film surface covered with randomly distributed 70-nm-high gold bumps showing small $(\ensuremath{\sim}0.7\ensuremath{\mu}\mathrm{m})$ and very bright $(\ensuremath{\sim}{10}^{3}$ times the background) spots. Wavelength and polarization dependencies of both positions and intensities of these SH bright spots as well as their statistics are investigated and compared for two areas having different nominal densities of scatterers, i.e., 25 and $50\ensuremath{\mu}{\mathrm{m}}^{\ensuremath{-}2}.$ For relatively large signals, it is found that, the probability density function of the SH signal follows the power-law dependence with the index being in the range of 2.5--3 for both values of the scattering density. We observe that, for incident laser powers in the range 3--40 mW, the SH bright spots exhibit, as expected, quadratic dependencies of the maximum signal on the incident power. However, for higher power levels, the interrogating laser beam causes nonlocal modifications of SH images, i.e., some SH bright spots disappear and others emerge even outside of the area that was exposed to a high power radiation. We relate the observed feature to surface plasmon polariton contribution in the process of multiple scattering, resulting in the formation of resonant eigenmodes (at fundamental and SH frequency) whose excitation in turn gives rise to the SH bright spots.

Kenneth Melvin Evenson

Kenneth Melvin Evenson

Direct observation of localized second-harmonic enhancement in random metal nanostructures.

Second harmonic (SH) scanning optical microscopy in reflection is used to image the gold film surface covered with randomly placed scatterers. SH images obtained with a tightly focused tunable (750-830 nm) laser beam show small (approximately 0.7 microm) and very bright (approximately 10(3) times the background) spots, whose locations depend on the wavelength and polarization of light. Comparing SH and fundamental harmonic (FH) images, we conclude that the localized SH enhancement occurs due to the overlap of FH and SH eigenmodes. The probability density function of the SH signal is found to follow the power-law dependence.

A tunable doughnut laser beam for cold-atom experiments

A doughnut-shaped light beam has been generated from a Gaussian mode (TEM00). Upon splitting the pump beam into two equal-intensity components and introducing unequal convergences with a phase delay while they are recombined, a doughnut mode results. The output beam is tunable and has a long propagation length. The evanescent field generated by a 360 mW (at 780 nm wavelength) beam creates an optical field of 600 nm decay length with a 5.75 neV repulsive dipole potential. Thus cold Rb atoms (at a temperature of 10 μK or less) released from a magneto-optical trap can be reflected by the funnel surface, with the result that the atoms are ultimately collected at the bottom.

Isotope Shifts in the TiO B–X (1–0) Band

The absorption spectrum of the B3Π–X3Δ (1–0) band of the TiO molecule has been studied at sub-Doppler resolution by crossing an effusive beam of TiO with a cw tunable laser beam and by collecting the induced fluorescence light. The five bands involving the 46Ti, 47Ti, 48Ti, 49Ti, and 50Ti isotopic species have been observed and interpreted. A simultaneous analysis of the five isotopic data leads to accurate molecular constants and energy levels for the B3Π electronic state.

Lithium Hydrides and Lithium Dimers: Sub‐Doppler Spectroscopy and Dynamics

AbstractDeep insights into molecular structures and dynamics were obtained for lithium dimers and lithium hydrides by sub‐Doppler spectroscopy combining molecular beams and cw tunable laser beams. For 7Li7Li species, accurate spectroscopic studies of the B 1Πu‐X 1Σg+ system were carried out, with the measurement of hundreds of wave numbers and with the determination of dissociation rates due to tunneling through the potential barrier of the B 1πu state. They led to the full description of the B state potential, including its barrier. For 6Li7Li species, weak but non‐negligible dissociation rates in the B 1Πu state were observed, measured and interpreted assuming that an electronic coupling between the B 1Πu and 1 1Πg states takes place through an ungerade‐gerade symmetry breaking. For LiH species, the A 1Σ+‐X 1 Σ+ system was revisited for both isotopomers. For this molecule, dynamical effects are predicted in highly‐excited Rydberg states, that could be studied by use of similar sub‐Doppler techniques.

Predissociation induced by ungerade-gerade symmetry breaking in theB1Πustate of the6Li7Limolecule

Unlike the ${}^{6}{\mathrm{Li}}^{6}\mathrm{Li}$ and ${}^{7}{\mathrm{Li}}^{7}\mathrm{Li}$ isotopomers, the ${}^{6}{\mathrm{Li}}^{7}\mathrm{Li}$ molecule exhibits surprisingly high dissociation rates for the levels that lie just above the dissociation limit of the $B{}^{1}{\ensuremath{\Pi}}_{u}$ state. As shown by Cacciani and Kokoouline [Phys. Rev. Lett. 84, 5296 (2000)] they can be attributed to a coupling between the $B{}^{1}{\ensuremath{\Pi}}_{u}$ and $1{}^{1}{\ensuremath{\Pi}}_{g}$ states, through the breaking of the ungerade-gerade symmetry. Systematic measurements of these dissociation rates have been performed by discriminating atomic fluorescence from molecular fluorescence in a sub-Doppler experiment involving a beam of lithium dimers and a cw tunable laser beam. Calculations of dissociation rates have been performed in the framework of a perturbative approach, by solving the coupled Schr\odinger equations through the use of the mapped Fourier grid representation method. Measured and calculated dissociation rates are in excellent agreement for a unique value of the coupling strength $h=0.144$ ${\mathrm{cm}}^{\ensuremath{-}1}.$

Sub-Doppler Spectroscopy of the LiH Molecule: The A–X System

The sub-Doppler analysis of the (A1Σ+–X1Σ+) system is performed for 6LiH and 7LiH by crossing an effusive beam of LiH by a cw tunable laser beam in the near UV; 384 lines are measured between 357 and 385 nm and assigned to 6LiH and 7LiH with an accuracy of 5 × 10−3 cm−1. Accurate Dunham coefficients and their mass-dependent corrections are deduced for the A1Σ+ electronic state of both isotopomers from the combination of previously published data and these new ones.

Variation of the linewidth of the A0+←X 1Σ+ transition of NaI

A continuous wave (cw) tunable single mode laser beam was propagated perpendicular to a collimated effusive molecular beam of NaI, and the excitation spectra of the A0+(v′,J′)−X 1Σ+(v″,J″) transitions of the NaI molecule were measured by monitoring the intensity of the molecular fluorescence. A large variation of the linewidth and line intensity with the rotational quantum number J was observed. The linewidths of rotationally resolved lines, attributable to lifetime broadening originating from predissociation, were obtained by a fitting of the observed spectral line shape to a Voigt profile.

Precise wave-number calibration in molecular spectra: the case of TiΩ

The spectrum of the titanium monoxide molecule has been studied by means of a sub-Doppler experiment combining an effusive beam of TiO and a cw tunable dye laser beam. In the 560 nm to 565 nm range, 260 absorption lines have been recorded through the laser-induced fluorescence technique of detection. Because the absorption profiles are narrow, a sensitive program of data acquisition and analysis is used. These wave-numbers have been measured by reference to the absorption spectrum of the iodine molecule to an absolute uncertainty of 30 MHz (0,001 cm-1). For the 140 lines of the c1-a1 (0-0) band of 48Ti16O which have been identified, the rms error between measured and calculated wave-numbers is just 21 MHz (0,0007 cm-1).

Rayleigh Scattering of Excimer Laser Light from Some Simple Molecules at 193 nm and 248 nm: The Effect of Polarization upon Imaging Diagnostics

Tunable excimer laser beams are increasingly used for imaging of combustion and/or flow systems. We (1) describe the need to use known polarization phenomena in interpreting diagnostics based on Rayleigh scattering (RS); (2) present relative RS cross sections for some important species at the most relevant wavelengths, 193 and 248 nm; and (3) measure the degree of linear polarization and the locking efficiency of the excimer beam, which are important diagnostic parameters that are otherwise difficult to obtain. Laser sheets, whose direction of linear polarization are adjustable, pass through gases. The Rayleigh light goes into intensified charge-coupled dervice (CCD) cameras. We report RS cross sections, normalized to those for N2, at 193 nm for H2, O2, H2O, CO2, and CH4, and at 248 nm for Ar, H2, O2, H2O, CO2, and CH4. Even when these normalized RS cross sections are used, the diagnostics can be very sensitive to the laser beam's polarization state. For example, for CO2, the normalized cross section changes by a factor of 4 with polarization. This usually unwanted sensitivity is greatest when an unmodified tunable excimer laser is used. The degree of polarization of the lasers, as well as their locking efficiency, is derived via RS from the spherical molecules Ar and CH4.

The TiOc1Φ–a1Δ (0–0) Band at Sub-Doppler Spectral Resolution: Spin–Orbit Interactions in thecState

Thec1Φ–a1Δ (0–0) band of titanium monoxide has been studied at sub-Doppler resolution by crossing a beam of TiO molecules with a cw tunable laser beam and collecting the laser-induced fluorescence. The rotational structures of theP,Q, andRbranches have been observed up to rotational quantum numbers equal to 52, 94, and 96, respectively. The spectroscopic data have been reduced to a set of 11 effective molecular constants with a root mean squared values of the residuals equal to 7 × 10−4cm−1. However, the necessity of including seventh order terms in the rotational development of the energy put into evidence a very strong spin–orbit interaction between thec1Φ (v= 0) andC3Δ3(v= 2 andv= 3) levels. The derived spin–orbit parameter allows for the reproduction of the measured wavenumbers without the introduction of higher order rotational terms into the energy development. The value of the spin–orbit parameter indicates the necessity of revisiting the configuration scheme of the involved electronic states.

Thermally assisted fluorescence of laser-excited OH A^2Σ^+ as a flame diagnostic tool

A simple laser-induced-fluorescence measurement technique for turbulent flame temperature and OH concentration measurement is proposed and successfully tested. The main idea is that a narrow-band tunable excimer laser beam (λ = 308 nm) is focused into a turbulent atmospheric-pressure nonpremixed flame. The OH molecule Q(1)(3) (X(2)II υ″ = 0, A(2)Σ+υ″ = 0) transition is excited. By subsequent vibrational energy transfer, the distribution in the molecules' upper electronic energy level (excited Σ(+) state) is redistributed. By evaluating the spectrum of the broadband emission from the υ' = 1 ? υ″ = 0 and υ' = 2 ? υ″ = 1 bands with a full spectral fit, we were able to evaluate the temperature and the OH molecule density. The concurrent processes of quenching, vibrational energy transfer, and rotational energy transfer were taken into account in the evaluation process. The results were compared with numerical flame calculations and revealed good agreement. One problem with this new proposed application of laser-induced fluorescence is the self-absorption of the emitted light. This, however, is shown not to be serious, but it has to be checked carefully. The main advantages are a simple experimental setup and procedure, high signal intensity, and a simple and straightforward data evaluation method. Thus this measurement technique is suitable for turbulent flame temperature and OH concentration measurement, and it is an alternative to other well-established techniques that are much more complicated.

Two studies using a tunable diode laser with frequency controlled by a stepping Michelson interferometer

The interference pattern obtained from a tunable diode laser beam through a stepping Michelson interferometer allows active control of its emission frequency. Two illustrations are given: (a) the N 2O concentration (≈ 100 ppbv) in stratospheric air samples has been measured with a precision of 3%. (b) the R(23) line of the CO fundamental band is analysed from a very low pressure of pure gas to pressures of about 1 atm in a CO-Xe gas mixture. Doppler, Dicke confinement and speed-dependent collision broadening effects act successively when the gas pressure increases. The narrowing parameter obtained by fitting the hard collision model (Rautian Sobel'man profile) to the observed line at pressures ⩽ 40 Torr gives a value of the diffusion coefficient close to the value deduced from the transport properties of gas mixtures.