We observe Doppler-free two-photon absorption spectra of three bands of S₁ ← S₀ transition of naphthalene. We use an optical frequency comb stabilized to a GPS clock as a frequency reference of a scanning cw laser. The use of the optical frequency comb enables us to decide transition frequencies of rovibronic lines and their linewidths with uncertainties of several tens of kHzA. Nishiyama, K. Nakashima, A. Matsuba, M. Misono, J. Mol. Spectrosc. 318, 40 (2015). We discuss the interactions in vibronic excited states of naphthalene based on the dependences of frequency shifts and linewidths on vibrational and on rotational quantum numbers. A. Nishiyama, K. Nakashima, A. Matsuba, M. Misono, J. Mol. Spectrosc. 318, 40 (2015)..

Dual-comb spectroscopies have great potential for high-resolution molecular and atomic spectroscopies, thanks to the broadband comb spectrum consisting of dense narrow modesA. Nishiyama, S. Yoshida, Y. Nakajima, H. Sasada, K. Nakagawa, A. Onae, K. and Minoshima, Opt. Express 24, 25894 (2016). In this study, we apply the dual-comb system to Doppler-free two-photon absorption spectroscopy. The outputs of two frequency combs excite several two-photon transitions of rubidiumA. Hipke, S. A. Meek, T. Ideguchi, T.W. Hänsch, and N. Picqué, Phys. Rev. A 90, 011805(R) (2014). and we obtained broadband Doppler-free spectra from dual-comb fluorescence signals. The fluorescence detection scheme circumvents the sensitivity limit which is effectively determined by the dynamic range of photodetectors in absorption-based dual-comb spectroscopies. Our system realized high-sensitive, Doppler-free high-resolution and broadband atomic spectroscopy. r0pt Figure A part of observed spectra of 5S_1/2 - 5D_5/2 transition is shown in the figure. The hyperfine structures of the F" = 1 - F′ = 3,2,1 transitions are fully-resolved and the spectral widths are approximately 5 MHz. The absolute frequency axis is precisely calibrated from comb mode frequencies which were stabilized to a GPS-disciplined clock. This work was supported by JST through the ERATO MINOSHIMA Intelligent Optical Synthesizer Project and Grant-in-Aid for JSPS Fellows (16J02345).

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Footnotes:

A. Nishiyama, S. Yoshida, Y. Nakajima, H. Sasada, K. Nakagawa, A. Onae, K. and Minoshima, Opt. Express 24, 25894 (2016).. A. Hipke, S. A. Meek, T. Ideguchi, T.W. Hänsch, and N. Picqué, Phys. Rev. A 90, 011805(R) (2014).,

We report a new laser spectroscopic technique capable of detecting weak state-state interactions in diatomic molecules. Specifically, a weak interaction has been observed between the 6pσ antibonding orbital of the CsXe (B ²Σ⁺_\frac12) state and a 5dσ MO associated with a 5dΛ (Λ = 0, 1) state. Thermal Cs-rare gas collision pairs are photoexcited by a circularly-polarized optical field having a wavelength within the B ²Σ⁺_\frac12 ← X ²Σ⁺_\frac12 (free←free) continuum. Subsequent dissociation of the B ²Σ⁺_\frac12 transient diatomic selectively populates the F = 4, 5 hyperfine levels of the Cs 6p ²P_\frac32 state, and circularly-polarized (σ⁺) amplified spontaneous emission (ASE) is generated on the Cs D₂ line. The dependence of Cs 6p spin polarization on the Cs(6p)-Xe internuclear separation (R), clearly shows an interaction between the CsXe(B ²Σ⁺_\frac12) state and a 5dΛ (Λ = 0, 1) potential of the diatomic molecule.

Probing the structure of a molecule as a chemical reaction unfolds has been a long standing goal in chemical physics. Most spectroscopic and diffraction techniques work well when the molecules are cold and thus vibrational motion is minimized. Yet, the very ability of a molecule to undergo structural changes implies that a significant amount of energy resides within the molecule. In order to probe structures of even medium sized molecules on an ultrafast time scale a technique that is sensitive to the molecular structure, yet insensitive to the vibrational motion is required. In our research we demonstrated that Rydberg electrons are remarkably sensitive to the molecular structure. Photoionization of a molecule out of Rydberg states reveals a purely electronic spectrum which is largely insensitive to vibrational motion. The talk illustrates how Rydberg electrons can serve as a probe for ultrafast structural dynamics in polyatomic molecules. The talk also demonstrates that photoionization through Rydberg states can be utilized for non-intrusive detection of polyatomic combustion intermediates in flames.

Core-nonpenetrating Rydberg states of molecules are a relatively untapped resource in molecular physics. Due to the l(l+1)/r² centrifugal barrier, the Rydberg electron in high-l states is essentially decoupled from the ion-core. This decoupling leads to the system becoming atom-like, with long lifetimes, an “almost good” l quantum number, and “pure-electronic” transitions that follow ∆J⁺=0 and ∆v⁺=0 propensity rules. Access to these nonpenetrating states is generally blocked by the necessity that the multistep excitation scheme traverses a “zone of death” in which nonradiative decay mechanisms are prohibitively fast. Coherent population transfer methods, such as STImulated Raman Adiabatic Passage (STIRAP), allow population of core-nonpenetrating states without even transiently populating states in the “zone of death.” We demonstrate coherent two-photon population transfer to Rydberg states of barium atoms using a pulsed dye laser and a chirped-pulse millimeter-wave spectrometer. Numerical calculations, using a density matrix formalism, reproduce our experimental results and provide insights into the fractional population transferred, optimal experimental conditions, and possibilities for future improvements, in particular extension to full STIRAP.

Extreme ultraviolet (XUV) spectroscopy is an inner shell technique that probes the M_2,3-edge excitation of atoms. Absorption of the XUV photon causes a 3p→3d transition, the energy and shape of which is directly related to the element and ligand environment. This technique is thus element-, oxidation state-, spin state-, and ligand field specific. A process called high-harmonic generation (HHG) enables the production of ultrashort ( ∼ 20fs) pulses of collimated XUV photons in a tabletop instrument. This allows transient XUV spectroscopy to be conducted as an in-lab experiment, where it was previously only possible at accelerator-based light sources. Additionally, ultrashort pulses provide the capability for unprecedented time resolution ( ∼ 50fs IRF). This technique has the capacity to serve a pivotal role in the study of electron and energy transfer processes in materials and chemical biology. I will present the XUV transient absorption instrument we have built, along with ultrafast transient M_2,3-edge absorption data of a series of small inorganic molecules in order to demonstrate the high specificity and time resolution of this tabletop technique as well as how our group is applying it to the study of ultrafast electronic dynamics of coordination complexes.

M-edge spectroscopy of first row transition metals (3p to 3d excitation) is the low energy analogue of more well-known K- and L-edge spectroscopy, but can be implemented without the use of a synchrotron. Instead, M-edge spectroscopy can be performed as a tabletop method, relying on high harmonic generation (HHG) to produce ultrashort ( ∼  20 fs) pulses of extreme ultraviolet (XUV) light in the range of 10-100s of eV. We have shown tabletop M-edge spectroscopy to be a valuable tool in determining the electronic structure of metal-centered coordination complexes and have demonstrated its capacity to yield element-specific information about a compound’s oxidation state, spin state, and ligand field. The power of this technique to distinguish these features makes it a promising addition to the arsenal of methods used to study metal-centered catalysts. A catalytic reaction can be initiated photochemically and the XUV probe can be used to track oxidative and structural changes to identify the key intermediates. Until recently tabletop XUV spectroscopy has been performed on thin film samples, but in order to examine homogeneous catalysis, the technique must be adapted to look at samples in the liquid phase. The challenges of adapting tabletop XUV spectroscopy to the liquid phase lie in the lower attenuation length of XUV light compared to soft and hard x-rays and the lower flux compared to synchrotron methods. As a result, the sample must be limited to a sub-micron thickness as well as isolated from the vacuum environment required for x-ray spectroscopy. I am developing a liquid flow cell that relies on confining the sample between two x-ray transmissive SiN membranes, as has been demonstrated for use at synchrotrons, but adapted to the unique difficulties encountered in tabletop XUV spectroscopy.

Methyl ammonium lead iodide (perovskite) is a leading candidate for next-generation solar cell devices. However, the fundamental photophysics responsible for its strong photovoltaic qualities are not fully understood. Ultrafast extreme ultraviolet (XUV) spectroscopy was used to investigate relaxation dynamics in perovskite with carrier specific signals arising from transitions from the common inner-shell level (I 4d) to the valence and conduction bands. Ultrashort (30 fs) pulses of XUV radiation in a broad spectrum (40-70 eV) were obtained using high-harmonic generation in a tabletop instrument. Transient absorption measurements with visible pump and XUV probe directly observed the dynamics of charge carriers after above-band and band-edge excitation.

Last year we presented a way of liquid phase sensing for H₂O and D₂O samples using a side-polished-fiber (SPF) sensor. It is a setup to combine the advantages of Supercontinuum light source with fiber-loop sensing method to make liquid phase CEAS sensing easier and more reliable. After some calculation we found out that with a SPF sensor we could only make use of less than 0.2% of the light from Supercontinuum source, so we decided to make changes on sensors in order to make more light usable. Instead of a SPF or similar evanescent wave sensors, if the light can be guided through a sample directly in free space, we can get almost 100% of the light to be used. So we replaced our sensor by using a mirror and two fibers placed vertical to it side-by-side. The mirror reflects light from one fiber to the other. The free space coupling can make the most of our Supercontinuum source, and a much stronger signal is observed so far. We are now able to use our setup to monitor very low H₂O concentrations such as saturated H₂O solution in organics like CCl₄. Hopefully we can make our system more reliable in the future to make it use in more samples and lower concentrations.

Electro-optic frequency combs recently have been applied to a wide range of physical and spectroscopic measurements because of attributes including, simplicity, robustness, flexibility, phase coherence, and high spectral power density. As an illustrative example, I will focus upon multiplexed saturation spectroscopy of atomic potassium (³⁹K) using ultra-high resolution frequency combs which contain up to a million individual teeth with spacings between 2 kHz and 2 MHz. Through the use of a self-heterodyne detection method, we have been able to simultaneously observe phenomena such as hole burning, hyperfine pumping, and electromagnetically induced transparency. I will discuss these measurements as well as future applications in molecular and atomic spectroscopy.

The application of electro-optic frequency combs to direct absorption spectroscopyD.A. Long et al., Opt. Lett. 39, 2688 (2014)as increased research interest in high-agility, modulator-based comb generation. This talk will review common architectures for electro-optic frequency comb generators as well as describe common self-heterodyne and multi-heterodyne (i.e., dual-comb) detection approaches. In order to achieve a sufficient signal-to-noise ratio on the recorded interferogram while allowing for manageable data volumes, broadband electro-optic frequency combs require deep coherent averaging,A.J. Fleisher et al., Opt. Express 24, 10424 (2016)referably in real-time. Applications such as cavity-enhanced spectroscopy, precision atomic and molecular spectroscopy, as well as time-resolved spectroscopy will be introduced.

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Footnotes:

D.A. Long et al., Opt. Lett. 39, 2688 (2014)h A.J. Fleisher et al., Opt. Express 24, 10424 (2016)p