Jack K. Steehler

Molecular, multiresonant coherent four-wave mixing spectroscopy

Abstract Recent research has expanded the capabilities of four-wave mixing by providing it with component selectivity, site selectivity, and mode selectivity. The selectivity is achieved by taking advantage of the three resonance enhancements that occur in a four-wave mixing process. New spectral scanning strategies allow one to scan a single resonance while maintaining the other two resonances at constant values. The constant resonances can be used to select a specific component, a specific site within an inhomogeneously broadened envelope of a component, and/or a specific vibrational or vibronic mode of that site. The scanned resonance will then contain enhanced features corresponding to the particular component, site, and/or mode that was chosen by the constant resonances. These component and site selective capabilities of the four-wave mixing complement the single vibronic level fluorescence methods. The relative transition intensities from a specific component or site reflect the mode coupling betwee...

Site-Selective Nonlinear Four-Wave Mixing by Multiply Enhanced Nonparametric and Parametric Spectroscopy

A family of nonlinear four-wave mixing techniques that are capable of site-selective organic spectroscopy are presented. Three lasers are used in the methods in order to achieve fully resonant mixing. Three lasers are shown to provide better sensitivity, selectivity, and versatility in the study of ground and excited electronic state vibrational spectroscopy. New approaches become possible in the establishment of resonances that translate the output signal from the normal Stokes or anti-Stokes side of the lasers to intermediate positions that are free of fluorescence interference. These new methods are divided into Multiply Enhanced Parametric Spectroscopy (MEPS) and Multiply Enhanced Nonparametric Spectroscopy (MENS), depending upon the spectroscopic characteristics for site-selective applications. The characteristics of MEPS and MENS are found to be quite different and depend upon the number and separation of the sites, the power of the lasers, the relative shifts of the levels, and the correlation effects in the inhomogeneous broadening. The feasibility of MENS and the site-selective capability of both CARS and MENS is demonstrated experimentally with the use of the pentacene:p-terphenyl system as a model.

Multiply enhanced site selective CARS spectroscopy

Three tunable lasers are used to perform site selective coherent anti‐Stokes Raman (CARS) spectroscopy in pentacene:p‐terphenyl crystals at 2 K. The vibrational and vibronic levels of pentacene in the individual substitutional sites of p‐terphenyl can be selectively examined by matching the resonances of a particular site. The signals from a site and the selectivity between sites are much higher than CARS performed with two tunable lasers. The spectra are dominated by lines from processes where the vibrational and vibronic states involved in the four‐wave mixing are associated with the same molecular mode. There are also important lines from processes where different vibrational and vibronic modes are involved in the nonlinear mixing. The intensities of lines from the resonant site are strongly dependent on the laser energy because of saturation of the electronic transition.

Parametric and nonparametric four‐wave mixing in pentacene: p‐terphenyl

Three tunable lasers are used to generate a fully resonant nonparametric process that generates new coherent output at a higher energy than fluorescence interference in condensed phase molecular systems. The pentacene:p‐terphenyl system is used as a model to test the basic ideas of the method and to compare the results with comparable spectra obtained with a parametric process. The resonance enhancement is shown to reflect intermode coupling between different vibrational and vibronic modes. Interference is observed between transitions of different pentacene sites that reflects the differences in the parametric and nonparametric processes. These differences become the dominant characteristic determining the potential for four‐wave nonlinear mixing methods to provide resolution within inhomogeneously broadened electronic bands.

Two-Frequency CARS for Mixture Analysis Using Pentacene in p -Terphenyl as a Model

The usefulness of multiresonant nonlinear spectroscopy for analyzing complicated mixtures requires understanding of the factors controlling the relative signal positions and intensities of individual components. In this study the effects of different types of detuning from resonance, of the interactions of such detunings, and of the incident laser powers are illustrated. CARS spectra specific to the different pentacene sites in p-terphenyl are observed for different types of detunings from resonance, and the complications introduced by multiple components are interpreted. Population changes at moderate laser powers due to resonance with the allowed S0 to S1 transition lead to striking changes in the observed spectra, and markedly affect the interpretation of the observed resonances.

Comparisons between 2D doubly vibrationally enhanced four wave mixing and site selective spectroscopy

Abstract We have constructed a nonlinear spectroscopic system for performing multiresonant four-wave mixing with infrared lasers. The system consists of three coherent sources, two of which are tunable in the infrared region of the spectrum. The sources are tuned to different vibrational resonances and the four-wave mixing output is monitored as a function of the two infrared frequencies. When the frequencies match direct infrared absorption or Raman transitions, the four-wave mixing output is enhanced. A two-dimensional display of the data shows the output intensity as a function of the two infrared frequencies. We observe that cross-peaks appear in the 2D spectra when multiple resonances are excited. We have named the method “doubly vibrationally enhanced four-wave mixing (DOVE-FWM)”. This method represents the long sought optical analogue to 2D nmr. It should provide a method that is complementary to nmr because of the difference in the time scales of the dephasing processes. Spin-lattice interactions fix the dephasing times for NMR measurements at millisecond time scales so nmr senses the ensemble average of a materials structure. Vibrational dephasing times occur on the picosecond time scale so the DOVE–FWM measurement represents a more instantaneous measurement of material structure.

Book Review of Principles of Molecular Photochemistry—An Introduction

This article presents a review of Principles of Molecular Photochemistry−An Introduction by Nicholas J. Turro, V. Ramamurthy, and J. C. Scaiano.

Introduction to Mass Spectrometry: Instrumentation, Applications, and Strategies for Data Interpretation, 4th Edition (by J. Throck Watson and O. David Sparkman)

Spectrochemical Analysis by Atomic Absorption and Emission is a comprehensive overview of atomic absorption and emission techniques, theory, and specialized applications, while Introduction to Mass Spectrometry: Instrumentation, Applications, and Strategies for Data Interpretation can serve as a textbook.

Ewing's Analytical Instrumentation Handbook, 3rd Edition (Cazes, Jack)

This books 31 chapters and more than 1000 large-size pages provide an enormous amount of information. About a third of the book discusses spectroscopic techniques; the second third covers various separation methods; and the remaining third hits a cross section of other topics, including four chapters on electrochemistry and three chapters on automation.

Exploring Chemical Analysis, 3rd Edition (Daniel C. Harris)

Exploring Chemical Analysis is a lighter version of Daniel Harris’s popular Quantitative Chemical Analysis textbook, which is a common choice for quantitative analysis courses.