Theory of femtosecond coherent anti-Stokes Raman scattering spectroscopy of gas-phase transitions

A theoretical analysis of coherent anti-Stokes Raman scattering (CARS) spectroscopy of gas-phase resonances using femtosecond lasers is performed. The time-dependent density matrix equations for the femtosecond CARS process are formulated and manipulated into a form suitable for solution by direct numerical integration (DNI). The temporal shapes of the pump, Stokes, and probe laser pulses are specified as an input to the DNI calculations. It is assumed that the laser pulse shapes are 70 fs Gaussians and that the pulses are Fourier-transform limited. A single excited electronic level is defined as an effective intermediate level in the Raman process, and transition strengths are adjusted to match the experimental Raman polarizability. The excitation of the Raman coherence is investigated for different Q-branch rotational transitions in the fundamental 2330 cm(-1) band of diatomic nitrogen, assuming that the pump and Stokes pulses are temporally overlapped. The excitation process is shown to be virtually identical for transitions ranging from Q2 to Q20. The excitation of the Raman coherences is also very efficient; for laser irradiances of 5x10(17) W/m2, corresponding approximately to a 100 microJ, 70 fs pulse focused to 50 microm, approximately 10% of the population of the ground Raman level is pumped to the excited Raman level during the impulsive pump-Stokes excitation, and the magnitude of the induced Raman coherence reaches 40% of its maximum possible value. The theoretical results are compared with the results of experiments where the femtosecond CARS signal is recorded as a function of probe delay with respect to the impulsive pump-Stokes excitation.     

利用时间分辨相干反斯托克斯拉曼散射技术研究光催化产氢反应动力学

基于飞秒再生放大器及飞秒光学参量放大器输出的激光脉冲, 搭建了宽带时间分辨相干反斯托克斯拉曼散射(CARS)测试装置, 并利用该装置研究了氢气与空气混合气体中氢气的相对含量, 探测相对延时与CARS光谱之间的关系. 通过调整延时, 获得了无非共振背景干扰的氢气CARS信号. 实验中测得的CARS信号强度与氢气浓度(分压)的平方呈良好的线性关系, 符合CARS理论预测. 同时测得的实验数据的信噪比表明: 在当前的实验条件下, 在氢气与空气混合气的总压为0.1 MPa时, 该装置可以对氢气的浓度进行测量, 且其检测极限可低至0.2%. 本文还利用该装置对三联吡啶苯乙炔Pt 配合物-Co 配合物-三乙醇胺(TEOA)的三元化学催化体系的产氢动力学行为进行了研究, 通过改变pH值讨论了该催化体系的产氢动力学机制. 结果表明过高的质子浓度会降低体系的产氢效率, 这可能是因为在酸性条件下, 作为质子和电子供体的三乙醇胺分解被抑制, 电子供应中断, 导致产氢反应的停止.     

Chirped coherent anti-stokes Raman scattering for high spectral resolution spectroscopy and chemically selective imaging

We describe a simple multiplex vibrational spectroscopic imaging technique based on employing chirped femtosecond pulses in a coherent anti-Stokes Raman scattering (CARS) scheme. Overlap of a femtosecond Stokes pulse with chirped pump/probe pulses introduces a temporal gate that defines the spectral resolution of the technique, allowing single-shot acquisition of high spectral resolution CARS spectra over a several hundred wavenumber bandwidth. Simulated chirped (c-) CARS spectra match the experimental results, quantifying the dependence of the high spectral resolution on the properties of the chirped pulse. c-CARS spectromicroscopy offers promise as a simple and generally applicable high spatial resolution, chemically specific imaging technique for studying complex biological and materials samples.     

Influence of electronic resonances on mode selective excitation with tailored laser pulses

Femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) gives access to ultrafast molecular dynamics. However, the gain of the temporal resolution entails a poor spectral resolution due to the inherent spectral width of the femtosecond excitation pulses. Modifications of the phase shape of one of the exciting pulses results in dramatic changes of the mode distribution reflected in coherent anti-Stokes Raman spectra. A feedback-controlled optimization of specific modes making use of phase and/or amplitude modulation of the pump laser pulse is applied to selectively influence the anti-Stokes signal spectrum. The optimization experiments are performed under electronically nonresonant and resonant conditions. The results are compared and the role of electronic resonances is analyzed. It can be clearly demonstrated that these resonances are of importance for a selective excitation by means of phase and amplitude modulation. The mode selective excitation under nonresonant conditions is determined mainly by the variation of the spectral phase of the laser pulse. Here, the modulation of the spectral amplitudes only has little influence on the mode ratios. In contrast to this, the phase as well as amplitude modulation contributes considerably to the control process under resonant conditions. A careful analysis of the experimental results reveals information about the mechanisms of the mode control, which partially involve molecular dynamics in the electronic states.     

Optimal coherent control of coherent anti-Stokes Raman scattering: signal enhancement and background elimination

The ability to enhance resonant signals and eliminate the non-resonant background is analyzed for coherent anti-Stokes Raman scattering (CARS). The analysis is done at a specific frequency as well as for broadband excitation using femtosecond pulse-shaping techniques. An appropriate objective functional is employed to balance resonant signal enhancement against non-resonant background suppression. Optimal enhancement of the signal and minimization of the background can be achieved by shaping the probe pulse alone while keeping the pump and Stokes pulses unshaped. In some cases analytical forms for the probe pulse can be found, and numerical simulations are carried out for other circumstances. It is found that a good approximate optimal solution for resonant signal enhancement in two-pulse CARS is a superposition of linear and arctangent-type phases for the pump. The well-known probe delay method is shown to be a quasi-optimal scheme for broadband background suppression. The results should provide a basis to improve the performance of CARS spectroscopy and microscopy.     

Imaging receptor-mediated endocytosis with a polymeric nanoparticle-based coherent anti-stokes Raman scattering probe

Coherent anti-Stokes Raman scattering (CARS) microscopy was used to visualize receptor-mediated endocytosis and intracellular trafficking with the aid of a CARS probe. The probe was made of 200-nm polystyrene particles encapsulated in folate-targeted liposomes. By tuning (omega(p) - omega(s)) to 3045 cm(-1), which corresponds to the aromatic C-H stretching vibration, the polystyrene nanoparticles with a high density of aromatic C-H bonds were detected with a high signal-to-noise ratio, while the epi-detected CARS signal from cellular organelles was cancelled by the destructive interference between the resonant contribution from the aliphatic C-H vibration and the nonresonant contribution. Without any photobleaching, the CARS probe allowed single-particle tracking analysis of intracellular endosome transport. No photodamage to cells was observed under the current experimental conditions. These results show the advantages and potential of using a CARS probe to study cellular processes.     

Coherent anti-Stokes Raman Scattering Microscopy.

Coherent anti-Stokes Raman scattering (CARS) microscopy is presented as a new nonlinear optical technique. The combination of vibrational spectroscopy and microscopy allows highly sensitive investigations of unlabelled samples. CARS is an ideal tool for studying a broad variety of samples. The main drawback of the technique is its non-zero-background nature, which implies that the signal has to be detected against a nonresonant background. The need to solve this problem is reflected in the rapid technological developments that have been observed during the last decade. Recent results show that CARS microscopy has the potential to become an important complementary technique that can be used with other well-established microscopic methods. Although it has some limitations, it offers unique access to many problems that cannot be tackled with conventional techniques. For this reason, it can be expected that the impressive growth of the field will continue.     

Coherent anti-Stokes Raman scattering: Spectroscopy and microscopy

In this review the basis, recent developments and applications of coherent anti-Stokes Raman scattering (CARS) in the fields of spectroscopy and microscopy are dialed with. The nonlinear susceptibility of the investigated molecule induced by pump and Stokes laser beams employed in the CARS technique is discussed. The relation between the nonlinear susceptibility, the different CARS laser intensities and the phase matching condition between them is also presented. The structure of CARS spectrum is analyzed as a function of the physical characteristics of the different employed lasers. This includes laser half widths, interference effects, cross-coherence and saturation of the resultant CARS signal by stimulated Raman scatter process (SRS). The different broadening mechanisms for CARS spectral line such as pressure and Doppler broadening are demonstrated. The recent progress in CARS for the in situ reaction flame diagnosis due to its suitability for detection of vibrational-rotational excited gas molecules present in the electronic ground state is discussed. CARS diagnosis for liquid- and solid-phases including the progress in polymeric materials is considered. The applications of CARS microscopy are reviewed in the view of its recent advances to study chemical and biological systems.     

Quantum theory of time-resolved femtosecond stimulated Raman spectroscopy: direct versus cascade processes and application to CDCl3

We present a quantum mechanical wave packet treatment of time-resolved femtosecond stimulated Raman spectroscopy (FSRS), or two-dimensional (2D) FSRS, where a vibrational coherence is initiated with an impulsive Raman pump which is subsequently probed by FSRS. It complements the recent classical treatment by Mehlenbacher et al. [J. Chem. Phys. 131, 244512 (2009)]. In this 2D-FSRS, two processes can occur concurrently but with different intensities: a direct fifth-order process taking place on one molecule, and a cascade process comprising two third-order processes on two different molecules. The cascade process comprises a parallel and a sequential cascade. The theory is applied to the 2D-FSRS of CDCl(3) where calculations showed that: (a) the cascade process is stronger than the direct fifth-order process by one order of magnitude, (b) the sidebands assigned to C-Cl E and A(1) bends, observed on both sides of the Stokes C-D stretch frequency, are not due to anharmonic coupling between the C-D stretch and the C-Cl bends, but are instead due to the coherent anti-Stokes Raman spectroscopy (CARS) and coherent Stokes Raman spectroscopy (CSRS) fields produced in the first step of the cascade process, (c) for each delay time between the femtosecond impulsive pump and FSRS probe pulses, the line shape of the sidebands shows an inversion symmetry about the C-D stretch frequency, and this is due to the 180(°) phase difference between the CARS and CSRS fields that produced the left and right sidebands, and (d) for each sideband, the line shape changes from positive Lorentzian to dispersive to negative Lorentzian, then to negative dispersive and back to positive Lorentzian with the period of the bending vibration, and it is correlated with the momentum of the wave packet prepared on the ground-state surface by the impulsive pump along the sideband normal coordinate.     

Time- and frequency-resolved coherent anti-Stokes Raman scattering spectroscopy with sub-25 fs laser pulses

In general, many different diagrams can contribute to the signal measured in broadband four-wave mixing experiments. Care must therefore be taken when designing an experiment to be sensitive to only the desired diagram by taking advantage of phase matching, pulse timing, sequence, and the wavelengths employed. We use sub-25 fs pulses to create and monitor vibrational wavepackets in gaseous iodine, bromine, and iodine bromide through time- and frequency-resolved femtosecond coherent anti-Stokes Raman scattering (CARS) spectroscopy. We experimentally illustrate this using iodine, where the broad bandwidths of our pulses, and Boltzmann population in the lower three vibrational levels conspire to make a single diagram dominant in one spectral region of the signal spectrum. In another spectral region, however, the signal is the sum of two almost equally contributing diagrams, making it difficult to directly extract information about the molecular dynamics. We derive simple analytical expressions for the time- and frequency-resolved CARS signal to study the interplay of different diagrams. Expressions are given for all five diagrams which can contribute to the CARS signal in our case.     

Surface-enhanced femtosecond CARS spectroscopy (SE-CARS) on pyridine

Surface-enhanced Raman scattering (SERS) has become an integral part of spectroscopy. The inelastic scattering process is enhanced by several orders of magnitude when molecules are in close contact to nano-structured coin metals. However, the use of surface enhancement in combination with nonlinear spectroscopy is by far not as common as in linear spectroscopy even though a more drastic effect could be expected. In our work, we report the observations we made from the preliminary studies on surface enhancement mechanisms in combination with coherent anti-Stokes Raman scattering (CARS) using femtosecond laser pulses. Silver colloids were used as enhancement medium. Molecules, which show conventional SERS were selected for the experiments. Femtosecond CARS was performed on these molecular systems in the presence and absence of silver colloids. The scattered CARS signal was collected both in the forward and sideward directions. From the analysis of the results general observations were made about the factors affecting the performance of SE-CARS.     

Femtosecond time resolved coherent anti-Stokes Raman spectroscopy: experiment and modelization of speed memory effects on H2-N2 mixtures in the collision regime

With the aim of temperature diagnostic, femtosecond time-resolved CARS (coherent anti-Stokes Raman spectroscopy) is applied to probe H2 in H2-N2 mixtures. In a first part, a Lorentzian profile is used to model the femtosecond CARS response. A difference between the experimental broadening and the expected one is observed in the collision regime. The observed broadening increases strongly in an inhomogeneous way with respect to the perturber concentration. This is of considerable importance for temperature measurements. In a second part, we show that in the collision regime, this inhomogeneous broadening is due to the speed dependence of the collisional parameters and the memory effects of the radiator speed. A new modelization of the time-resolved CARS response taking into account the speed memory effects is presented and applied to the temperature diagnostic in H2-N2 mixtures. The numerical results are in good agreement with experiments.     

High-pressure ammonia adsorption and dissociation on clean fe(111) and oxygen-precovered fe(111) studied by sum frequency generation vibrational spectroscopy

The adsorption of gases N2, H2, O2, and NH3 that play a role in ammonia synthesis have been studied on the Fe(111) crystal surface by Sum Frequency Generation (SFG) vibrational spectroscopy using an integrated ultrahigh vacuum/high-pressure system. SFG spectra are presented for the dissociation intermediates, NH2 ( approximately 3325 cm-1) and NH ( approximately 3235 cm-1) under high pressure of ammonia (200 Torr) on the clean Fe(111) surface. Addition of 0.5 Torr of oxygen to 200 Torr of ammonia does not significantly change the bonding of dissociation intermediates to the surface. However, it leads to a phase change of nearly 180 degrees between the resonant and nonresonant second-order nonlinear susceptibility of the surface, demonstrated as a reversal of the SFG spectral features. Heating the surface in the presence of 200 Torr of ammonia and 0.5 Torr of oxygen reduces the oxygen coverage, which can be seen from the SFG spectra as another relative phase change of 180 degrees . The reduction of the oxide is also supported by Auger electron spectroscopy. The result suggests that the phase change of the spectral features could serve as a sensitive indicator of the chemical environment of the adsorbates. Clean Fe(111) is found to have a large SFG nonresonant signal. The magnitude of the nonresonant signal was dependent on the adsorption species; O2 and N2 decrease, while H2 and NH3 increase the SFG nonresonant signal. The change in nonresonant signal is correlated to the change in work function for Fe(111) upon adsorption. Adsorption-induced changes in the SFG nonresonant signal was used as an indicator of surface conditions and to monitor surface reactions.     

Control of nitromethane photoionization efficiency with shaped femtosecond pulses

The applicability of adaptive femtosecond pulse shaping is studied for achieving selectivity in the photoionization of low-density polyatomic targets. In particular, optimal dynamic discrimination (ODD) techniques exploit intermediate molecular electronic resonances that allow a significant increase in the photoionization efficiency of nitromethane with shaped near-infrared femtosecond pulses. The intensity bias typical of high-photon number, nonresonant ionization is accounted for by reference to a strictly intensity-dependent process. Closed-loop adaptive learning is then able to discover a pulse form that increases the ionization efficiency of nitromethane by ～150%. The optimally induced molecular dynamics result from entry into a region of parameter space inaccessible with intensity-only control. Finally, the discovered pulse shape is demonstrated to interact with the molecular system in a coherent fashion as assessed from the asymmetry between the response to the optimal field and its time-reversed counterpart.     

Direct measurement of S-branch N(2)-H(2) Raman linewidths using time-resolved pure rotational coherent anti-Stokes Raman spectroscopy

S-branch N(2)-H(2) Raman linewidths have been measured in the temperature region 294-1466 K using time-resolved dual-broadband picosecond pure rotational coherent anti-Stokes Raman spectroscopy (RCARS). Data are extracted by mapping the dephasing rates of the CARS signal temporal decay. The J-dependent coherence decays are detected in the time domain by following the individual spectral lines as a function of probe delay. The linewidth data set was employed in spectral fits of N(2) RCARS spectra recorded in binary mixtures of N(2) and H(2) at calibrated temperature conditions up to 661 K using a standard nanosecond RCARS setup. In this region, the set shows a deviation of less than 2% in comparison with thermocouples. The results provide useful knowledge for the applicability of N(2) CARS thermometry on the fuel-side of H(2) diffusion flames.     

CARS diagnostics of acetylene formation in a plasma test reactor for hydrocarbon synthesis

Measurements of acetylene concentrations in an arc plasma test reactor using coherent anti-Stokes Raman scattering (CARS) are described. The method is based on the measurement of relative spectral shapes in the presence of a substantial nonresonant background. Calculations of CARS spectra, necessary for the concentration analysis, are described. The quality of calculated spectra is examined with the aid of reference spectra taken in a graphite oven operated under variable, known temperatures and concentrations. The measurements with the plasma test reactor yield spatially resolved concentration distributions of a few percent. Comparison with 1R absorption measurements shows satisfactory agreement.     

Interaction of coadsorbed CH3Cl and D2O layers on Pd(111) studied by sum frequency generation

Adsorption of methyl chloride and coadsorption of CH3Cl and D2O on Pd(111) surfaces at T=100 K have been studied under ultrahigh-vacuum conditions using femtosecond sum frequency generation (SFG) spectroscopy in the spectral regions of CH and OD bands. On the bare Pd(111) substrate, the CH3Cl coverage dependence of the resonant SFG signal is consistent with a progressive molecular rearrangement starting at half saturation followed by the growth of two ordered monolayers in which the molecular axes are perpendicular to the surface. When CH3Cl is adsorbed on top of predeposited D2O on Pd(111), the SFG signals as a function of the CH3Cl exposure indicate that methyl chloride is adsorbed onto D2O through hydrogen bonding. On the contrary when the adsorption order is reversed the strong decrease of the CH3 signal as a function of the D2O exposure is explained by assuming that water molecules penetrate inside the CH3Cl layers, leading to the formation of disordered CH3Cl clusters. In all cases a nonresonant contribution due to molecular adsorption is observed and it shows a dependence upon surface structure and coverage significantly different from that of the resonant vibrational bands.     

Resonance multiplex cars of fluorescing acridines

Resonance enhanced coherent anti-Stokes Raman scattering (CARS) spectra have been obtained for the highly fluorescing acridine dyes, acridine orange and proflavine, in dilute methanol solutions at submillimolar concentrations. Spectra have also been taken in the multiplex mode by the use of a broad-band Stokes laser and a Vidicon OMA detection system. Several Raman bands are observed in the 1100–1600 cm^?1 region originating from the acridine ring modes. Upon decreasing the beam crossing angle a continuous transition from the normal CARS spectrum to a negative spectrum in the nonresonant background is observed.     

Theory of ultrafast nonresonant multiphoton transitions in polyatomic molecules: basics and application to optimal control theory

A systematic approach is presented to describe nonresonant multiphoton transitions, i.e., transitions between two electronic states without the presence of additional intermediate states resonant with the single-photon energy. The method is well suited to describe femtosecond spectroscopic experiments and, in particular, attempts to achieve laser pulse control of molecular dynamics. The obtained effective time-dependent Schrodinger equation includes effective couplings to the radiation field which combine powers of the field strength and effective transition dipole operators between the initial and final states. To arrive at time-local equations our derivation combines the well-known rotating wave approximation with the approximation of slowly varying amplitudes. Under these terms, the optimal control formalism can be readily extended to also account for nonresonant multiphoton events. Exemplary, nonresonant two- and three-photon processes, similar to those occurring in the recent femtosecond pulse-shaping experiments on CpMn(CO)(3), are treated using related ab initio potential energy surfaces.