Molecular Transition Frequencies Research Articles

Plasmon-Exciton Interaction in Perspective Hetero-Systems

Surface plasmons and excitons have been widely studied experimentally and theoretically for various material systems. However, a number of aspects require further deeper study and understanding, among which the connection of these quasi-particles occupies an important place. New physical effects arise when plasmons and excitons in nanostructures begin to be localized at certain small distances, as a result, we can talk about their coupling. Complex systems containing the excitation of plasmons and excitons, as well as their coupling, show interesting optical properties that they cannot exhibit individually. In this type of system, the plasmon enhances the coupling between the system and the external field, and the exciton controls certain spectral properties, which opens up new possibilities for tuning their optical response. The transferred energy between plasmons and excitons becomes an important factor affecting their interaction when the resonance frequency of the localized plasmon is very close to the molecular energy transition frequency. Two types of coupling can occur depending on the ratio between the strength of the coupling and the energy losses of individual components in the system, namely strong and weak. In addition to the mutual coupling between the plasmon and the exciton, their different linewidths and ability to couple to an external field provide a variety of means to tune the optical properties of hybrid systems. Thus, it enables precise control of light at the nanometer scale, opening up possibilities for new electronics and photonics applications. In this review, we highlight the features of weak and strong modes of plasmon-exciton coupling, modern trends, and perspectives in the study of hetero-systems semiconductor–metal, metal–2D material, semiconductor–molecule, etc. Semiconductor-metal hybrid nanostructures open up exciting opportunities for the study of quantum phenomena, optical processes, and multiparticle interactions and confidently lead to application in new photonics devices.

Search for Dark-Matter-Induced Oscillations of Fundamental Constants Using Molecular Spectroscopy.

A possible implication of an ultralight dark matter field interacting with the standard model degrees of freedom is oscillations of fundamental constants. Here, we establish direct experimental bounds on the coupling of an oscillating ultralight dark matter field to the up, down, and strange quarks and to the gluons, for oscillation frequencies between 10 and 10^{8} Hz. We employ spectroscopic experiments that take advantage of the dependence of molecular transition frequencies on the nuclear masses. Our results apply to previously unexplored frequency bands and improve on existing bounds at frequencies >5 MHz. We also improve on the bounds for coupling to the electromagnetic field and the electron field, in particular spectral windows. We identify a sector of ultralight dark matter and standard model coupling space where thebounds from equivalence principle tests may be challenged by next-generation experiments of the present kind.

Coupling, lifetimes, and "strong coupling" maps for single molecules at plasmonic interfaces.

The interaction between excited states of a molecule and excited states of a metal nanostructure (e.g., plasmons) leads to hybrid states with modified optical properties. When plasmon resonance is swept through molecular transition frequency, an avoided crossing may be observed, which is often regarded as a signature of strong coupling between plasmons and molecules. Such strong coupling is expected to be realized when 2|⟨U⟩|/ℏΓ > 1, where ⟨U⟩ and Γ are the molecule-plasmon coupling and the spectral width of the optical transition, respectively. Because both ⟨U⟩ and Γ strongly increase with decreasing distance between a molecule and a plasmonic structure, it is not obvious that this condition can be satisfied for any molecule-metal surface distance. In this work, we investigate the behavior of ⟨U⟩ and Γ for several geometries. Surprisingly, we find that if the only contributions to Γ are lifetime broadenings associated with the radiative and nonradiative relaxation of a single molecular vibronic transition, including effects on molecular radiative and nonradiative lifetimes induced by the metal, the criterion 2|⟨U⟩|/ℏΓ > 1 is easily satisfied by many configurations irrespective of the metal-molecule distance. This implies that the Rabi splitting can be observed in such structures if other sources of broadening are suppressed. Additionally, when the molecule-metal surface distance is varied keeping all other molecular and metal parameters constant, this behavior is mitigated due to the spectral shift associated with the same molecule-plasmon interaction, making the observation of Rabi splitting more challenging.

Unlocking synchrotron sources for THz spectroscopy at sub-MHz resolution.

Synchrotron radiation (SR) has proven to be an invaluable contributor to the field of molecular spectroscopy, particularly in the terahertz region (1-10 THz) where its bright and broadband properties are currently unmatched by laboratory sources. However, measurements using SR are currently limited to a resolution of around 30 MHz, due to the limits of Fourier-transform infrared spectroscopy. To push the resolution limit further, we have developed a spectrometer based on heterodyne mixing of SR with a newly available THz molecular laser, which can operate at frequencies ranging from 1 to 5.5 THz. This spectrometer can record at a resolution of 80 kHz, with 5 GHz of bandwidth around each molecular laser frequency, making it the first SR-based instrument capable of sub-MHz, Doppler-limited spectroscopy across this wide range. This allows closely spaced spectral features, such as the effects of internal dynamics and fine angular momentum couplings, to be observed. Furthermore, mixing of the molecular laser with a THz comb is demonstrated, which will enable extremely precise determinations of molecular transition frequencies.

Molecular transition frequencies of CO2 near 1.6 µm with kHz-level uncertainties

We present measurements of molecular transition frequencies based on the comb-locked cavity ring-down spectroscopy technique, reporting vacuum transition frequencies of Doppler-broadened 12C16O2 in the 1.6 μm region for the (30012) ← (00001) and (30013) ← (00001) bands with an average combined standard uncertainty of ≈ 1 kHz. A global four-state model was fit to these data and literature values to provide spectroscopic parameters and a best-case fit precision of 4 kHz. We identified and quantified an interaction between the (30012) and (33301) states which was manifest as an observed Fermi-resonance-type perturbation in the (30012) ← (00001) band. This interaction was accounted for in the global fit, substantially reducing systematic uncertainties in the spectroscopic parameters of the (30012) state and altering many predicted transition frequencies in the (30012) ← (00001) band by 1 MHz or more relative to literature values. The accurate transition frequencies reported here may be considered as SI-traceable reference data for diverse applications including remote sensing and telecommunications.

New bounds on macroscopic scalar-field topological defects from nontransient signatures due to environmental dependence and spatial variations of the fundamental constants

We point out that in models of macroscopic topological defects composed of one or more scalar fields that interact with standard model fields via scalar-type couplings the back action of ambient matter on the scalar field(s) produces an environmental dependence of the fundamental constants of nature as well as spatial variations of the fundamental constants in the vicinity of dense bodies such as Earth due to the formation of a ``bubblelike'' defect structure surrounding the dense body. In sufficiently dense environments, spontaneous symmetry breaking may be inhibited altogether for ${\ensuremath{\phi}}^{2}$ interactions, potentially delaying the cosmological production of topological defects to rather late times. We derive bounds on nontransient variations of the fundamental constants from torsion-pendulum experiments that search for equivalence-principle-violating forces, experiments comparing the frequencies of ground- and space-based atomic clocks as well as ground-based clocks at different heights in the recent Tokyo Skytree experiment, and measurements comparing atomic and molecular transition frequencies in terrestrial and low-density astrophysical environments. Our results constrain the present-day mass-energy fraction of the Universe due to a network of infinite domain walls produced shortly after the big bang nucleosynthesis or cosmic microwave background epochs to be ${\mathrm{\ensuremath{\Omega}}}_{\text{walls},0}\ensuremath{\ll}{10}^{\ensuremath{-}10}$ for the symmetron model with a ${\ensuremath{\phi}}^{4}$ potential and ${\ensuremath{\phi}}^{2}$ interactions, improving over cosmic microwave background quadrupolar temperature anisotropy bounds by at least 5 orders of magnitude. Our newly derived bounds on domain walls with ${\ensuremath{\phi}}^{2}$ interactions via their effects of nontransient variations of the fundamental constants are significantly more stringent than previously reported clock- and cavity-based limits on passing domain walls via transient signatures (regardless of their possible degradation due to previously neglected effects of the strongly repulsive potential generated by Earth on the passing domain walls) and previous bounds from different types of nontransient signatures (by about 10 orders of magnitude for wall thicknesses comparable to the size of Earth), under the same set of assumptions.

SI-traceable molecular transition frequency measurements at the 10−12 relative uncertainty level

Accurate and spectroscopic measurements of molecular transition frequencies are increasingly being employed in a variety of rigorous tests of physics, including the validity of quantum electrodynamics, the proton–electron mass ratio, and the dipole moment of the electron. Near-infrared molecular transitions may also underpin secondary frequency standards for length metrology and provide convenient wavelength standards for telecommunication and spectroscopy. To report progress in this field, we describe measurements of near-infrared Doppler-broadened carbon dioxide line positions. Our experiment uses cavity ring-down spectroscopy in the linear domain and provides an accurate International System of Units (SI)-traceable frequency axis via active locking of the probe laser and ring-down cavity to a Cs-clock-referenced optical frequency comb. The approach has several key attributes: the ability to rapidly scan the probe laser while maintaining a phase lock to the optical frequency comb, continuously tunable spectrum frequency axes that are not limited by the cavity mode spacing, and high signal-to-noise-ratio spectra resulting in stationary statistics amenable to long-term averaging. This performance is achieved by phase locking a single-mode probe laser to the optical frequency comb, coherent electro-optical phase modulation for rapid cavity mode-to-mode shifting of the probe laser, and sub-kilohertz-level frequency stabilization of the ring-down cavity relative to the probe laser. We report vacuum transition frequencies of five transitions in the ( 30012 ) ← ( 00001 ) vibrational band of 12 C 16 O 2 near 1.57 µm and achieve combined standard uncertainties as low as 212 Hz, corresponding to a relative combined standard uncertainty near 10 − 12 .

Attainable accuracies of QH+ rotational transition frequencies (Q: 40Ca, 24Mg, 202Hg)

Molecular rotational transition frequencies are useful as frequency standard in the terahertz region. The transition frequencies of QH+ molecular ions (Q: an even isotope of Group II element) in a linear trap are useful as measurement uncertainties are low. An uncertainty given by the Stark and Zeeman shifts can be below 10−15 using 202HgH+ molecular ion.

Effect of the Polymer Chain Arrangement on Exciton and Polaron Dynamics in P3HT and P3HT:PCBM Films

Arrangement of polymer chains at the interface with an acceptor is one of the key issues which influences the interfacial charge transfer. Films of P3HT and P3HT:PCBM blends with the different polymer chain arrangement have been prepared from one-component and binary solvents representing a mixture of chloroform and different poor solvents with high boiling temperature. Electronic absorption and photoluminescence spectra evidenced in favor of reduced disorder of P3HT films as a result of use of poor solvents; the ordering was displayed through structuring and narrowing the spectral bands, indicative of decreasing width of Gaussian distribution of molecular transition frequencies. At the same time, transient absorption of singlet excitons showed that exciton decay in highly ordered P3HT films slows down as compared to the disordered film, and this effect was reproduced for the different poor solvents used. A more pronounced effect was revealed in P3HT:PCBM blends where much faster decay of excitons was found in disordered as-prepared P3HT:PCBM film from chloroform as compared to the annealed film or films prepared from the binary solvents. A complex behavior of polarons in the different regions of the blend film, i.e., within crystalline domains of P3HT and at the interface of P3HT/PCBM, was observed. The conclusion was drawn that chain disorder induces easier exciton dissociation and charge transfer at the P3HT/PCBM interface.

Search for the Variation in (mp/me) Using Two Vibrational Transition Frequencies of Molecular Ions

We propose a method to search for the variation in the proton-to-electron mass ratio μ = (mp/me) based on the precise measurement of Q(v′) = [f(v′) − f(2v′)/2]/f(v′), where f(v′) is the 16O2+ \(X^{2}\Pi _{1/2}(v,J) = (0,1/2) \to (v',1/2)\) transition frequency. Q(v′) is proportional to μ−1/2, and it can be measured with an uncertainty of 10−18. This method is also applicable to other molecular transition frequencies (i) J = 0 → 0 or 1/2 → 1/2 transition, (ii) transition between stretched states, and (iii) the same sign for the light shift induced by the clock lasers in f(v′) and f(2v′).

Communication: Molecular near-infrared transitions determined with sub-kHz accuracy.

Precise molecular transition frequencies are needed in various studies including the test of fundamental physics. Two well isolated ro-vibrational transitions of 12C16O at 1.57 μm, R(9) and R(10) in the second overtone band, were measured by a comb-locked cavity ring-down spectrometer. Despite the weakness of the lines (Einstein coefficient A≃0.008 s-1), Lamb-dip spectra were recorded with a signal-to-noise ratio over 1000, and the line positions were determined to be 191 360 212 761.1 and 191 440 612 662.2 kHz, respectively, with an uncertainty of 0.5 kHz (δν/ν=2.6×10-12). The present work demonstrates the possibility to explore extensive molecular lines in the near-infrared with sub-kHz accuracy.

Absorption and photoluminescence in organic cavity QED

Organic microcavities can be engineered to reach exotic quantum regimes of strong and ultrastrong light-matter coupling. However, the microscopic interpretation of their spectroscopic signals can be challenging due to the competition between coherent and dissipative processes involving electrons, vibrations and cavity photons. We develop here a theoretical framework based on the Holstein-Tavis-Cummings model and a Markovian treatment of dissipation to account for previously unexplained spectroscopic features of organic microcavities consistently. We identify conditions for the formation of dark vibronic polaritons, a new class of light-matter excitations that are not visible in absorption but lead to strong photoluminescence lines. We show that photon leakage from dark vibronic polaritons can be responsible for enhancing photoluminescence at the lower polariton frequency, and also explain the apparent breakdown of reciprocity between absorption and emission in the vicinity of the bare molecular transition frequency. Successful comparison with experimental data demonstrates the applicability of our theory.

Accuracy estimation of the O216+ transition frequencies targeting the search for the variation in the proton-electron mass ratio

The precise measurement of molecular vibrational transition frequencies is useful in the search for variation in the proton-to-electron mass ratio {\mu}. Homonuclear diatomic molecules are particularly advantageous for precise measurement because of the small Stark shift. This paper discusses the Stark and Zeeman shifts in the 16O2+ vibrational transition frequencies.To search for the variation {\mu}(<10-17/yr), the X2{\Pi}1/2(v,J)=(0,1/2)-(v',1/2)(v'>1)in transition frequencies are much more advantageous than the transition frequencies between the X2{\Pi}1/2 v=21 and a4{\Pi}1/2 v=0 states, whose sensitivity to the variation in {\mu} is high because of the accidental degeneracy.

Absolute molecular transition frequencies measured by three cavity-enhanced spectroscopy techniques.

Absolute frequencies of unperturbed (12)C(16)O transitions from the near-infrared (3-0) band were measured with uncertainties five-fold lower than previously available data. The frequency axis of spectra was linked to the primary frequency standard. Three different cavity enhanced absorption and dispersion spectroscopic methods and various approaches to data analysis were used to estimate potential systematic instrumental errors. Except for a well established frequency-stabilized cavity ring-down spectroscopy, we applied the cavity mode-width spectroscopy and the one-dimensional cavity mode-dispersion spectroscopy for measurement of absorption and dispersion spectra, respectively. We demonstrated the highest quality of the dispersion line shape measured in optical spectroscopy so far. We obtained line positions of the Doppler-broadened R24 and R28 transitions with relative uncertainties at the level of 10(-10). The pressure shifting coefficients were measured and the influence of the line asymmetry on unperturbed line positions was analyzed. Our dispersion spectra are the first demonstration of molecular spectroscopy with both axes of the spectra directly linked to the primary frequency standard, which is particularly desirable for the future reference-grade measurements of molecular spectra.

Dynamical Casimir-Polder interaction between a chiral molecule and a surface

We develop a dynamical approach to study the Casimir-Polder force between a initially bare molecule and a magnetodielectric body at finite temperature. Switching on the interaction between the molecule and the field at a particular time, we study the resulting temporal evolution of the Casimir-Polder interaction. The dynamical self-dressing of the molecule and its population-induced dynamics are accounted for and discussed. In particular, we find that the Casimir-Polder force between a chiral molecule and a perfect mirror oscillates in time with a frequency related to the molecular transition frequency, and converges to the static result for large times.

Characterizing of variation in the proton-to-electron mass ratio via precise measurements of molecular vibrational transition frequencies

We propose measurements of v,N=0,0→(1,0) transition frequencies of alkali earth–alkali diatomic molecules and alkali earth-hydride molecular ions. We show that the vibrational transitions considered here are useful for detecting a possible variation in the proton–electron mass ratio, possibly at the 10−16 sensitivity level. In the experiments, cold alkali earth–alkali molecules, obtained by binding cold atoms or by molecular laser cooling, are trapped by laser light at a frequency, at which the Stark shift is eliminated. Alkali earth-hydride molecular ions are sympathetically cooled with another ion to form a string crystal at the position, at which the electric field is zero. The vibrational transitions are observed by the Raman transition induced by two lasers with frequencies, at which the Stark shift is eliminated.

Precision experiments and fundamental physics at low energies – Part I

Precision experiments and fundamental physics at low energies – Part I

Microwave and submillimeter molecular transitions and their dependence on fundamental constants

Microwave and submillimeter molecular transition frequencies between nearly degenerated rotational levels, tunneling transitions, and mixed tunneling‐rotational transitions show an extremely high sensitivity to the values of the fine‐structure constant, α, and the electron‐to‐proton mass ratio, μ. This review summarizes the theoretical background on quantum‐mechanical calculations of the sensitivity coefficients of such transitions to tiny changes in α and μ for a number of molecules which are usually observed in Galactic and extragalactic sources, and discusses the possibility of testing the space‐ and time‐invariance of fundamental constants through comparison between precise laboratory measurements of the molecular rest frequencies and their astronomical counterparts. In particular, diatomic radicals CH, OH, NH+, and a linear polyatomic radical C3H in Π electronic ground state, polyatomic molecules NH3, ND3, NH2D, NHD2, H2O2, H3O+, CH3OH, and CH3NH2 in their tunneling and tunneling‐rotational modes are considered. It is shown that sensitivity coefficients strongly depend on the quantum numbers of the corresponding transitions. This can be used for astrophysical tests of Einstein's Equivalence Principle all over the Universe at an unprecedented level of sensitivity of , which is a limit three to two orders of magnitude lower as compared to the current constraints on cosmological variations of α and μ: , .

Comb-linked, cavity ring-down spectroscopy for measurements of molecular transition frequencies at the kHz-level

We present a low uncertainty measurement technique for determining molecular transition frequencies. This approach is complementary to sub-Doppler saturation spectroscopies and is expected to enable new frequency measurements for a wide variety of molecular species with uncertainties at the kHz-level. The technique involves measurements of Doppler broadened lines using cavity ring-down spectroscopy whereby the probe laser is actively locked to the ring-down cavity and the spectrum frequencies are linked directly to an optical frequency comb that is referenced to an atomic frequency standard. As a demonstration we have measured the transition frequency of the (30012) ← (00001) P14e line of CO2 near 1.57 μm with a combined standard uncertainty of ~9 kHz. This technique exhibits exceptional promise for measurements of transition frequencies and pressure shifting parameters of many weak absorbers, and indicates the potential for substantially improved measurements when compared to those obtained with conventional spectroscopic methods.

Broadening of rovibrational absorption lines of carbon monoxide and dioxide molecules as a result of collisions with walls

The mechanisms determining the absorption line profiles of CO and СО2 molecules enclosed in cells with sizes changing from a few tens of nanometers to a few centimeters are considered. Frequencies of molecular transitions from several reciprocal centimeters (rotational transitions) to 2000 cm−1 (rovibrational transitions) are considered. Doppler and intermolecular collisional broadening and line broadening as a result of collisions with walls of the cell filled with the gas are discussed. It is demonstrated that the contribution of line broadening as a result of collisions with cell walls to the total line width can change from several fractions to several tens of percent depending on the cell size. The temperature dependence of the contributions of line broadening and their dependence on the absorption wavelength are discussed.