Absorption Line Shape Research Articles

Quantum cascade laser measurements of CH4 linestrength and temperature-dependent self-broadening and narrowing parameters at 7.16 µm

Absorption lineshapes for 10 CH4 transitions in the v2 and v4 band were measured at high-resolution in a thermodynamically-controlled optical cell using a quantum cascade laser near 7.16 µm. Linestrength, self-broadening, -narrowing, pressure-shift coefficients and their temperature dependence were recorded over a temperature and pressure range of 290–1050 K and 3–80 kPa, respectively. Voigt, Rautian, speed-dependent Voigt profiles were least-squares fit to the measured absorption spectra to determine the best-fit lineshape parameters. Significant collisional-narrowing and speed-dependent effects were observed but no line mixing behaviour was found in pure CH4 at the experimental conditions. The self-pressure-shift coefficients were found to change signs from negative to positive with increasing temperature and the Frost's formula fit well their temperature dependence for the studied transitions. The measured spectra provided tests of the CH4 line lists near 1397 cm−1 released in HITEMP 2020 and the recorded spectroscopic parameters compared well with the values in the database. Several weak transitions excluded from the HITEMP database were observed at elevated temperatures with the line positions given here. The analyses of uncertainties in all the measured spectroscopic parameters were also demonstrated in this work.

Calibration-free wavelength modulation spectroscopy based on even-order harmonics.

This paper proposes a novel and rapid calibration-free wavelength modulation spectroscopy algorithm based on even-order harmonics. The proposed algorithm, analytically deduced from Voigt line-shape function, only involves simple algebraic operations to describe the actual gas absorption spectra, thus eliminating the time-consuming simulations and line-shape fitting procedures adopted in traditional algorithms. Instead of acquiring the entirely scanned absorption line-shape, the proposed technique only requires extraction of the peak values of the harmonics. This characteristic significantly benefits gas diagnosis at elevated pressure and/or temperature, in which the entirely scanned absorption is very difficult to be obtained due to the broadened line-shapes. The proposed algorithm is validated by both numerical simulation and condition-controlled experiment, indicating millisecond-level calculation of gas parameters with the relative error less than 4% in the experiments.

Vibrational and vibronic structure of isolated point defects: The nitrogen-vacancy center in diamond

We present a theoretical study of vibrational and vibronic properties of a point defect in the dilute limit by means of first-principles density functional theory calculations. As an exemplar we choose the negatively charged nitrogen-vacancy center, a solid-state system that has served as a testbed for many protocols of quantum technology. We achieve low effective concentrations of defects by constructing dynamical matrices of large supercells containing tens of thousands of atoms. The main goal of the paper is to calculate luminescence and absorption lineshapes due to coupling to vibrational degrees of freedom. The coupling to symmetric $a_1$ modes is computed via the Huang-Rhys theory. Importantly, to include a nontrivial contribution of $e$ modes we develop an effective methodology to solve the multi-mode $E \otimes e$ Jahn-Teller problem. Our results show that for NV centers in diamond a proper treatment of $e$ modes is particularly important for absorption. We obtain good agreement with experiment for both luminescence and absorption. Finally, the remaining shortcomings of the theoretical approach are critically reviewed. The presented theoretical approach will benefit identification and future studies of point defects in solids.

Effect of spontaneously generated coherence (SGC) on the line shapes of absorption, transparency, dispersion and group index of a four-level inverted Y-type atom–lasers coupling system

Effect of spontaneously generated coherence (SGC) on the line shapes of absorption, transparency, dispersion and group index of a four-level inverted Y-type atom–lasers coupling system

A universal Urbach rule for disordered organic semiconductors

In crystalline semiconductors, absorption onset sharpness is characterized by temperature-dependent Urbach energies. These energies quantify the static, structural disorder causing localized exponential-tail states, and dynamic disorder from electron-phonon scattering. Applicability of this exponential-tail model to disordered solids has been long debated. Nonetheless, exponential fittings are routinely applied to sub-gap absorption analysis of organic semiconductors. Herein, we elucidate the sub-gap spectral line-shapes of organic semiconductors and their blends by temperature-dependent quantum efficiency measurements. We find that sub-gap absorption due to singlet excitons is universally dominated by thermal broadening at low photon energies and the associated Urbach energy equals the thermal energy, regardless of static disorder. This is consistent with absorptions obtained from a convolution of Gaussian density of excitonic states weighted by Boltzmann-like thermally activated optical transitions. A simple model is presented that explains absorption line-shapes of disordered systems, and we also provide a strategy to determine the excitonic disorder energy. Our findings elaborate the meaning of the Urbach energy in molecular solids and relate the photo-physics to static disorder, crucial for optimizing organic solar cells for which we present a revisited radiative open-circuit voltage limit.

Experimental and simulated study of line-shape models for measuring spectroscopic parameters using the WM-DAS method — Part II: Broadening and narrowing of select near-infrared [formula omitted] and CO lines perturbed by Ar, [formula omitted] and He

Wavelength modulation-direct absorption spectroscopy (WM-DAS) has been used to measure the absorption line shapes of the H2O transition at 7299.431 cm−1 and the CO transition at 4297.705 cm−1 perturbed by Ar, N2 and He. The foreign-gas broadening, narrowing and pressure-shift parameters were determined from various lineshape models at pressures from 1 to 100 kPa. The measured N2-broadening coefficients are the largest among the three perturbers and the measured N2- and Ar-pressure-shift coefficients are both negative while that for He is positive. The nonlinear pressure dependence of the velocity-changing rates for the Galatry and Rautian profiles and the speed-dependent coefficients for Speed-dependent Voigt profile were all observed for the 6 binary systems. The measured velocity-changing rates were all larger than the corresponding dynamic friction coefficients in H2O-Ar/N2/He while smaller than those in CO-Ar/N2/He. The narrowing parameters were also retrieved and compared by taking both the Dicke narrowing and speed dependent effects into account. The partially Correlated quadratic Speed-dependent Hard-Collision profile was simulated and fit by the other simplified profiles to further validate the nonlinear behaviour of the narrowing parameters, which is consistent with the measured results.

Time-resolved Low-resolution Spectra of BLAP-009 and BLAP-014

Abstract We present time resolved low resolution spectra of BLAP-009 and BLAP-014 collected with SPRAT on the Liverpool Telescope. The spectra were median-stacked in four ranges of phase. It was intended to note the variation in the shape of the helium absorption lines as a function of phase, but they do not appear to vary with time in low resolution spectra. The lack of clear offsets in the absorption lines suggest that there can only be small radial velocity due to either unseen companion or rapid expansion/contraction of the envelope.

Theory of Nonradiative Energy Transfer between Two Optical Ions Using Proper Adiabatic Approximation

Using the adiabatic approximation of polyatomic molecules and methods of molecular quantum electrodynamics, we have derived the Förster-Dexter theory of energy transfer between two optical ions. Although we have assumed a single configurational coordinate, the theory can be easily formulated for a multiple normal coordinate system. This approach leads smoothly to the energy transfer rates originally derived by Dexter1 for electric dipole-electric dipole interaction without any additional assumptions regarding the normalization of the wavefunctions in the energy space. Using the transfer rates thus derived, and the band shape functions, W p , a new function F(T) has been proposed to study the temperature dependence of energy transfer rates due to the overlap of the emission and absorption lineshapes of the donor and acceptor ions, respectively. This function has been studied for various scenarios of the relative alignment of the zero phonon lines of these ions in order to understand the participation of phonons in the energy transfer process. We have also compared this quantum mechanically derived F(T) function with that derived using semiclassical theory. The agreements are very good at high temperatures.

Optical Signals to Monitor the Dynamics of Phonon‐Modified Rabi Oscillations in a Quantum Dot

AbstractSemiconductor quantum dots are solid state few‐level systems which can interact strongly with light. As such, they can be used as a single photon source or to perform solid‐state quantum‐optics experiments. A major difference to atoms is the interaction with the lattice vibrations, that is, the phonons, which strongly affect the optical signals from a quantum dot. In this paper, we calculate the time‐resolved pump‐probe signals from a continuously driven quantum dot accounting for detuned excitation and electron–phonon interaction. We provide analytical equations giving insight into the different phenomena observed in the spectra like an oscillation between absorptive and dispersive line shapes. For detuned excitation, the electron–phonon interaction can lead to phonon‐assisted occupation of the exciton, giving rise to a strong asymmetry in the dynamical behavior depending on the sign of the detuning. The time‐resolved spectra monitor this relaxation path, which helps understanding the effect of electron–phonon interaction in quantum dots.

A line-of-sight channel model for the 100\u2013450 gigahertz frequency band

This paper documents a simple parametric polynomial line-of-sight channel model for 100–450 GHz band. The band comprises two popular beyond fifth generation (B5G) frequency bands, namely, the D band (110–170 GHz) and the low-THz band (around 275–325 GHz). The main focus herein is to derive a simple, compact, and accurate molecular absorption loss model for the 100–450 GHz band. The derived model relies on simple absorption line shape functions that are fitted to the actual response given by complex but exact database approach. The model is also reducible for particular sub-bands within the full range of 100–450 GHz, further simplifying the absorption loss estimate. The proposed model is shown to be very accurate by benchmarking it against the exact response and the similar models given by International Telecommunication Union Radio Communication Sector. The loss is shown to be within ±2 dBs from the exact response for one kilometer link in highly humid environment. Therefore, its accuracy is even much better in the case of usually considered shorter range future B5G wireless systems.

Understanding radiative transitions and relaxation pathways in plexcitons

Summary Molecular aggregates on plasmonic nanoparticles have emerged as attractive systems for the studies of polaritonic light-matter states, called plexcitons. Such systems are tunable, scalable, easy to synthesize, and offer sub-wavelength confinement, all while giving access to the ultrastrong light-matter coupling regime, promising a plethora of applications. However, the complexity of these materials prevented the understanding of their excitation and relaxation phenomena. Here, we follow the relaxation pathways in plexcitons and conclude that while the metal destroys the optical coherence, the molecular aggregate coupled to surface processes significantly contributes to the energy dissipation. We use two-dimensional electronic spectroscopy with theoretical modeling to assign the different relaxation processes to either molecules or metal nanoparticle. We show that the dynamics beyond a few femtoseconds has to be considered in the language of hot electron distributions instead of the accepted lower and upper polariton branches and establish the framework for further understanding.

Two-photon absorption line shapes in the transit-time limit.

A weak excitation transit-time resolution limited analytic line shape is derived for a Doppler broadening-free degenerate two-photon transition from a standing wave with a TEM00 transverse profile. This approximation is appropriate when the collisional mean free path is much larger than the transverse width of the TEM00 beam. It is considerably simpler than the two-photon absorption line shape previously published, Bordé, C. R. Hebd. Seances Acad. Sci., Ser. B 282, 341-344 (1976), which was derived for more general experimental conditions. The case of a saturating field, with an intensity-dependent shift of the resonance frequency, is treated and expressed in reduced units. Numerical calculations are presented for the line shape for a range of the reduced intensity and light intensity shifts values.

Electron thermalization and relaxation in laser-heated nickel by few-femtosecond core-level transient absorption spectroscopy

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel M$_{2,3}$ edge. It is observed that the core-level absorption lineshape of photoexcited nickel can be described by a Gaussian broadening ($\sigma$) and a red shift ($\omega_{s}$) of the ground state absorption spectrum. Theory predicts, and the experimental results verify that after initial rapid carrier thermalization, the electron temperature increase ($\Delta T$) is linearly proportional to the Gaussian broadening factor $\sigma$, providing quantitative real-time tracking of the relaxation of the electron temperature. Measurements reveal an electron cooling time for 50 nm thick polycrystalline nickel films of 640$\pm$80 fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of $\omega_{s}\propto\Delta T^{1.5}$. Rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4 fs photoexcitation pulse until the carriers reach a quasi-thermal equilibrium. Entwined with a <6 fs instrument response function, carrier thermalization times ranging from 34 fs to 13 fs are estimated from experimental data acquired at different pump fluences and it is observed that the electron thermalization time decreases with increasing pump fluence. The study provides an initial example of measuring electron temperature and thermalization in metals in real time with XUV light, and it lays a foundation for further investigation of photoinduced phase transitions and carrier transport in metals with core-level absorption spectroscopy.

Nonlinear features of Fano resonance: a QM/EM study.

The nonlinear Fano effects on the absorption of hybrid systems composed of a silver nanosphere and an indoline dye molecule have been systematically investigated by the hybrid approach, which combines the quantum mechanics method (QM) with the computational electromagnetic method (EM). The absorption spectra of the dye molecule in the proximity of an Ag nanoparticle have been calculated by changing the incident field intensity, the phenomenological dephasing of molecular excitation, and the enhancement ratio of the near field. The contribution of molecular nonlinear response properties and the quantum interferences of the incident and scattered fields and of resonant plasmon-molecular excitations to the spectra has been identified. It is in no doubt that Fano resonance due to the plasmon-molecular interaction can appear in both the weak and strong field regimes; however, the Fano effect is more pronounced in the strong field regime where quantum interference leads to a nonlinear Fano effect controlled by a complex field-dependent Fano factor. When the incident field is strong enough, the resonance antisymmetry structure is spectrally resolved, and it changes with the change of the field intensity. As the field intensity varies from weak to strong, the Fano lineshape's asymmetry increases with increasing intensity in the beginning, and then decreases with a further increase of the field intensity attributed to the increase of the detuning energy induced by the integrated energy shift upon field dressing during the excitation. Decreasing the enhancement ratio of the near field or the dephasing of molecular excitation can also control the spectral lineshape transformation from an asymmetric profile to a symmetric Lorentzian lineshape. These findings are consistent with previous experimental and theoretical observations arisen by quantum interferences and are expected to stimulate further work toward exploring the plasmon-molecular interplay and the applications of Fano resonance in optical switching and sensing.

The spectral-shapes of absorption, emission, ECD and CPL of a fluorene-fused [7]helicene: Vibronic effect and solvent inhomogenous broadening

We compute the line shapes of absorption (ABS), emission (EMI), electronic circular dichroism (ECD), and circularly polarized luminescence (CPL) of a fluorene-fused [7]helicene. To elucidate different physico-chemical factors responsible for the spectral shapes, we account for Franck-Condon (FC) contribution, Herzberg-Teller (HT) effect and Duschinsky rotation, as well as the solvent inhomogenous broadening. The latter is connected to the solvent reorganization energy which is calculated with the state-specific (SS) implementation of Polarizable Continuum Model (PCM). To understand the relative importance of different effects of equilibrium geometry displacements, frequencies changes, and Duschinsky mixing, we carefully compare and discuss the results of Adiabatic Hessian (AH), Adiabatic Shift and Frequencies (ASF), and Adiabatic Shift (AS) models in details. Our results agree with the experimental spectra very well. They show that FC contribution dominates the spectral shapes and HT effect leads to the break of mirror images between absorptive (ABS + ECD) and emissive (EMI + CPL) spectra.

Pulse length effects on autoionizing states under the influence of intense SASE XUV fields

The Fano absorption line shape of an autoionizing state encodes information on its internal atomic structure and dynamics. When driven near-resonantly with intense extreme ultraviolet (XUV) electric fields, the absorption profile can be deliberately modified, including observable changes of both the line-shape asymmetry and strength of the resonance, revealing information on the underlying dynamics of the system in response to such external driving. We report on the influence of the XUV pulse parameters at high intensity that can be achieved with a free-electron laser (FEL) with statistically broadened spectra based on self-amplified spontaneous emission (SASE). More specifically, the impact of the FEL pulse duration is studied for the example of the doubly excited 2s2p resonance in helium, where line-shape modifications have been measured with XUV transient absorption spectroscopy in Fraunhofer-type transmission geometry. A computational few-level-model provides insight into the impact of different average pulse durations of the stochastic FEL pulses. These findings are supported by measurements performed at the Free-Electron Laser in Hamburg (FLASH) and provide further insight into XUV strong-coupling dynamics of resonant transitions driven by intense high-frequency FEL sources.

Ultrafast dynamics in rubrene and its spectroscopic manifestation.

A multimode Brownian oscillator model is employed to investigate absorption line shapes of rubrene in solutions and thin films. Excellent agreement has been obtained between simulated and measured absorption spectra. Furthermore, using the parameters obtained from the fitting absorption spectra of rubrene, the dynamics of singlet fission is explored by the Dirac-Frenkel time-dependent variation with multiple Davydov trial states. By comparing the absorption spectra between a conical intersection model and the multimode Brownian oscillator model, the optimal system-bath coupling parameter of η = 0.2 is determined, and ensuing population dynamics of the S1 state is found to concur with a previously proposed coherent singlet fission mechanism. We further simulate the two-dimensional spectra to reveal the wavepacket dynamics on both excited state and ground state potential energy surfaces. It is demonstrated that the wavepacket motion of tuning modes plays a significant role in the evolution of two-dimensional spectra.

Stochastic scattering theory for excitation-induced dephasing: Time-dependent nonlinear coherent exciton lineshapes.

We develop a stochastic theory that treats time-dependent exciton-exciton s-wave scattering and that accounts for dynamic Coulomb screening, which we describe within a mean-field limit. With this theory, we model excitation-induced dephasing effects on time-resolved two-dimensional coherent optical lineshapes and we identify a number of features that can be attributed to the many-body dynamics occurring in the background of the exciton, including dynamic line narrowing, mixing of real and imaginary spectral components, and multi-quantum states. We test the model by means of multidimensional coherent spectroscopy on a two-dimensional metal-halide semiconductor that hosts tightly bound excitons and biexcitons that feature strong polaronic character. We find that the exciton nonlinear coherent lineshape reflects many-body correlations that give rise to excitation-induced dephasing. Furthermore, we observe that the exciton lineshape evolves with the population time over time windows in which the population itself is static in a manner that reveals the evolution of the multi-exciton many-body couplings. Specifically, the dephasing dynamics slow down with time, at a rate that is governed by the strength of exciton many-body interactions and on the dynamic Coulomb screening potential. The real part of the coherent optical lineshape displays strong dispersive character at zero time, which transforms to an absorptive lineshape on the dissipation timescale of excitation-induced dephasing effects, while the imaginary part displays converse behavior. Our microscopic theoretical approach is sufficiently flexible to allow for a wide exploration of how system-bath dynamics contribute to linear and non-linear time-resolved spectral behavior.

Modeling nonlocal electron-phonon coupling in organic crystals using interpolative maps: The spectroscopy of crystalline pentacene and 7,8,15,16-tetraazaterrylene.

Electron-phonon coupling plays a central role in the transport properties and photophysics of organic crystals. Successful models describing charge- and energy-transport in these systems routinely include these effects. Most models for describing photophysics, on the other hand, only incorporate local electron-phonon coupling to intramolecular vibrational modes, while nonlocal electron-phonon coupling is neglected. One might expect nonlocal coupling to have an important effect on the photophysics of organic crystals because it gives rise to large fluctuation in the charge-transfer couplings, and charge-transfer couplings play an important role in the spectroscopy of many organic crystals. Here, we study the effects of nonlocal coupling on the absorption spectrum of crystalline pentacene and 7,8,15,16-tetraazaterrylene. To this end, we develop a new mixed quantum-classical approach for including nonlocal coupling into spectroscopic and transport models for organic crystals. Importantly, our approach does not assume that the nonlocal coupling is linear, in contrast to most modern charge-transport models. We find that the nonlocal coupling broadens the absorption spectrum non-uniformly across the absorption line shape. In pentacene, for example, our model predicts that the lower Davydov component broadens considerably more than the upper Davydov component, explaining the origin of this experimental observation for the first time. By studying a simple dimer model, we are able to attribute this selective broadening to correlations between the fluctuations of the charge-transfer couplings. Overall, our method incorporates nonlocal electron-phonon coupling into spectroscopic and transport models with computational efficiency, generalizability to a wide range of organic crystals, and without any assumption of linearity.

Effect of incident laser sheet orientation on the OH-PLIF imaging of detonations

This study aims to investigate the effect of laser sheet orientation on the OH-planar laser-induced fluorescence (OH-PLIF) imaging of a detonation wave and to identify the potential benefit of using a transverse laser orientation, compared to the conventional frontal orientation. In this study, we developed a new laser-induced fluorescence (LIF) model, based on a preexisting one, to include a more fundamental calculation of the absorption lineshape and the effects of the laser orientation on the fluorescence signal. One- and two-dimensional simulations are used to validate this new LIF model for both laser orientations using OH-PLIF images from the literature. The effect of the two laser orientations on detonation imaging is investigated numerically on a two-dimensional OH field. Based on these results, the frontal orientation seems more suitable to collect information near the detonation front, while the transverse laser orientation can give more quantitative information far from the front and only for short optical paths. With the present laser configuration and experimental conditions, both laser orientations present the same limitation with a strong laser energy absorption for long optical paths. While these first results show a qualitative agreement with experimental data, new experimental data are required to quantitatively validate this promising transverse laser orientation and confirm its relevance for OH-PLIF imaging of detonations.