mid-IR Pump Research Articles

Perturbed free induction decay in ultrafast mid-IR pump–probe spectroscopy

We studied theoretically and experimentally, the perturbed free induction decay phenomenon for an anharmonic vibrational system in degenerate IR pump–probe experiments. We derived the theory for the perturbed free induction decay using perturbation theory in the interaction picture for the fundamental and overtone transitions. Degenerate broadband mid-IR (MIR) pump–probe experiments on W ( CO ) 6 were carried out to measure transient absorption spectra and kinetic signals at negative time delays, respectively. The experimental measurements were compared and fitted with the presented theory, showing excellent agreement.

Time-Domain SFG Spectroscopy Using Mid-IR Pulse Shaping: Practical and Intrinsic Advantages

Sum-frequency generation (SFG) spectroscopy is a ubiquitous tool in the surface sciences. It provides infrared transition frequencies and line shapes that probe the structure and environment of molecules at interfaces. In this article, we apply techniques learned from the multidimensional spectroscopy community to SFG spectroscopy. We implement balanced heterodyne detection to remove scatter and the local oscillator background. Heterodyning also separates the resonant and nonresonant signals by acquiring both the real and imaginary parts of the spectrum. We utilize mid-IR pulse shaping to control the phase and delay of the mid-IR pump pulse. Pulse shaping allows phase cycling for data collection in the rotating frame and additional background subtraction. We also demonstrate time-domain data collection, which is a Fourier transform technique, and has many advantages in signal throughput, frequency resolution, and line shape accuracy over existing frequency domain methods. To demonstrate time-domain SFG spectroscopy, we study an aryl isocyanide on gold, and find that the system has an inhomogeneous structural distribution, in agreement with computational results, but which was not resolved by previous frequency-domain SFG studies. The ability to rapidly and actively manipulate the mid-IR pulse in an SFG pules sequence makes possible new experiments and more accurate spectra.

Electronic Properties of N(5)-Ethyl Flavinium Ion

We investigated the electronic properties of N(5)-ethyl flavinium perchlorate (Et-Fl(+)) and compared them to those of its parent compound, 3-methyllumiflavin (Fl). Absorption and fluorescence spectra of Fl and Et-Fl(+) exhibit similar spectral features, but the absorption energy of Et-Fl(+) is substantially lower than that of Fl. We calculated the absorption signatures of Fl and Et-Fl(+) using time-dependent density functional theory (TD-DFT) methods and found that the main absorption bands of Fl and Et-Fl(+) are (π,π*) transitions for the S(1) and S(3) excited states. Furthermore, calculations predict that the S(2) state has (n,π*) character. Using cyclic voltammetry and a simplistic consideration of the orbital energies, we compared the HOMO/LUMO energies of Fl and Et-Fl(+). We found that both HOMO and LUMO orbitals of Et-Fl(+) are stabilized relative to those in Fl, although the stabilization of the LUMO level was more pronounced. Visible and mid-IR pump-probe experiments demonstrate that Et-Fl(+) exhibits a shorter excited-state lifetime (590 ps) relative to that of Fl (several nanoseconds), possibly due to faster thermal deactivation in Et-Fl(+), as dictated by the energy gap law. Furthermore, we observed a fast (23-30 ps) S(2) → S(0) internal conversion in transient absorption spectra of both Fl and Et-Fl(+) in experiments that utilized pump excitations with higher energy.

Nonequilibrium carriers in intrinsic graphene under interband photoexcitation

We study nonequilibrium carriers (electrons and holes) in an intrinsic graphene at low temperatures under far- and mid-infrared (IR) radiation in a wide range of its intensities. The energy distributions of carriers are calculated using a quasiclassic kinetic equation which accounts for the energy relaxation due to acoustic phonons and the radiative generation-recombination processes associated with thermal radiation and the carrier photoexcitation by incident radiation. It is found that the nonequilibrium distributions are determined by an interplay between weak energy relaxation on acoustic phonons and generation-recombination processes as well as by the effect of pumping saturation. Due to the effect of saturation, the carrier distribution functions can exhibit plateaus around the pumping region at elevated intensities. As shown, at sufficiently strong mid-IR pumping, the population inversion can occur below the pumping energy. The graphene dc conductivity as a function of the pumping intensity exhibits a pronounced nonlinearity with a sub-linear region at fairly low intensities and a saturation at a strong pumping. However, an increase in the pumping intensity in very wide range leads only to a modest increase in the carrier concentration and, particularly, the dc conductivity. The graphene conductivity at mid-IR irradiation exhibit strong sensitivity to mechanisms of carrier momentum relaxation.

Femtosecond mid-IR pump–probe spectroscopy of photoinduced insulator to metal transition in dimer Mott insulator κ-(BEDT-TTF) 2X

Ultrafast dynamics of the photoinduced insulator (I) to metal (M) transition were investigated using femtosecond mid-infrared pump–probe spectroscopy in two-dimensional organic Mott insulators [bis (ethylenedithio)]-tetrathiafulvalene (BEDT-TTF) salts κ-(BEDT-TTF) 2X ( κ-(ET) 2X, where X denotes anion). In κ-( d-ET) 2Cu[N(CN) 2]Br, a metallic state was photogenerated using a phonon-mediated mechanism: the effective bandwidth increases through the photoinduced molecular rearrangement. The mechanism differs fundamentally from the previously reported photoinduced filling control in one-dimensional Mott insulators.

Luminous HC3N line and the HCN/HCO+ ratio in NGC 4418

Aims. We investigate the properties of the nuclear molecular gas and address the nature of the deeply buried source driving the IR emission of NGC 4418. Methods. We present IRAM 30 m observations and basic non-LTE, single component radiative transport modelling of HNC, HCN, HCO + , CN, HC3N, and H2CO Results. We find that NGC 4418 has a rich molecular chemistry – including unusually luminous HC3N J = 10–9, 16–15, and 25–24 line emission – compared to the other high density tracers. We furthermore detect: ortho-H2CO 2–1, 3–2; CN 1–0, 2–1; HCO + , 1–0. 3–2, HCN 3–2, HNC 1–0, 3–2, and tentatively OCS 12–11. The HCN, HCO + ,H 2CO, and CN line emission can be fitted to densities of n = 5× 10 4 −10 5 cm −3 and gas temperatures Tk = 80–150 K. Both HNC and HC3N are, however, significantly more excited than the other species, which requires higher gas densities or radiative excitation through mid-IR pumping. The HCN line intensity is fainter than that of HCO + and HNC for the 3–2 transition, in contrast to previous findings for the 1–0 lines, where the HCN emission is the most luminous. Assuming all line emission is emerging from the same gas, abundances of the observed species are estimated to be similar to each other within factors of 2–5. The most noteworthy of these is the high abundance of HC3N and a small-to-moderate abundance ratio between HCN and HCO + . Conclusions. We tentatively suggest that the observed molecular line emission is consistent with a young starburst, where the emission can be understood as emerging from dense, warm gas with an additional PDR component. We find that X-ray chemistry is not required to explain the observed mm-line emission, including the HCN/HCO + 1–0 and 3–2 line ratios. The luminous HC3N line emission is an expected signature of dense, starforming gas. A deeply buried AGN cannot be excluded, but its impact on the surrounding molecular medium is then suggested to be limited. However, detailed modelling of HC3N abundances in X-ray dominated regions (XDRs) should be carried out. The possibility of radiative excitation should also be investigated further.

Vibrational Substructure in the OH Stretching Transition of Water and HOD

Ultrafast nonlinear vibrational spectroscopy with mid-IR pumping and incoherent anti-Stokes Raman probing is used to study v = 1 excitations of OH stretching (νOH) of water and of HOD in D2O solvent (HOD/D2O). The parent νOH decay and the appearance of daughter stretching and bending excitations are simultaneously monitored, which allows for characterization of the stretch decay pathways. At all times and with all pump frequencies within the νOH band, the excited-state spectrum can be fit by two overlapping subbands, a broader red-shifted band and a narrower blue-shifted band . We show these subbands are dynamically distinguishable. They decay with different lifetimes and evidence characteristically different decay pathways. Excitations of the subband generate bending vibrations that does not. The shorter lifetime (∼0.5 ps) of the subband compared to the subband (0.8−0.9 ps) results primarily from enhanced stretch-to-bend anharmonic coupling. The subbands represent persistent structures in the excited state, in that interconversion between subbands (2−10 ps) is slower than excited-state decay. A tentative structural interpretation is proposed. The subband, on the basis of simulations, its red shift ,and its shorter lifetime, is proposed to result from strongly hydrogen-bonded “ice-like” water. The subband has a smaller amplitude in HOD/D2O than in water, possibly because HOD has a single localized OH-stretching vibration whereas water has two delocalized stretching vibrations.

Vibrational energy relaxation pathways of water

Vibrational energy relaxation (VR) of the OH stretch ν OH and bend δ H 2O in water is studied by the mid-IR pump with anti-Stokes Raman probe technique. The broad ν OH band in water consists of two inhomogeneously broadened subbands. VR in the larger red-shifted subband ν OH R, with T 1=0.55 ps, is shown to occur by the mechanism ν OH→ δ H 2O (1/3) and ν OH → ground state (2/3). VR in the smaller longer-lived blue-shifted subband ν OH B, with T 1=0.75 ps, occurs by the mechanism ν OH → ground state. The bending fundamental δ H 2O decays directly to the ground state with T 1=1.4 ps.

Vibrational energy redistribution in polyatomic liquids: 3D infrared–Raman spectroscopy

Vibrational energy relaxation (VER) in polyatomic liquids and solutions can be studied in unprecedented detail using ultrafast incoherent anti-Stokes Raman spectroscopy to monitor the decay of a parent vibration generated with a tunable mid-IR pump pulse, and the appearance of daughter excitations at lower frequencies. A 2D vibrational spectroscopy technique results from fixing the mid-IR pump pulse and obtaining a time series of anti-Stokes Raman spectra. A 3D technique results from obtaining 2D spectra as a function of pump frequency. This 3D technique is presently the most powerful method available for studying VER in condensed phases. It provides information on VER mechanisms and pathways, liquid-state structural relaxation resulting in spectral diffusion, excitation transfer among sites with different local structures, strong correlations between vibrations as a result of Fermi resonance, and extremely weak anharmonic interactions resulting in VER via multistep vibrational cascades. The technique is illustrated by recent results obtained with ambient temperature associated liquids water (and deuterated water) and methanol, and by new measurements of strong correlations between CH stretching and bending and OH stretching and bending vibrations in methanol.

Three-Dimensional Spectroscopy of Vibrational Energy Relaxation in Liquid Methanol

Vibrational energy relaxation (VER) of neat methanol at ambient temperature is studied with a three-dimensional vibrational spectroscopy technique. The first two dimensions are represented by a time series of incoherent anti-Stokes Raman spectra at a given mid-IR pump frequency. The third dimension involves changing the mid-IR pump frequency within the manifold of CH and OH stretching transitions ν(CH) and ν(OH). Each 2D representation shows the VER dynamics occurring as a result of pumping a specific vibrational transition. The decay of the pumped transition into first-generation daughters, and the subsequent decay into second-, and even third-generation daughters can be monitored. The third dimension shows how VER depends on the nature of the pumped transition. Additional information about the buildup of excitation in the bath of lower-energy collective excitations (phonons) is obtained by monitoring the heating of CCl4 spectators in methanol−CCl4 mixtures. Three distinct stages are seen in VER of ν(CH). The pumped ν(CH) creates two quanta of bending vibrations δ(CH) or δ(OH) (0−1 ps), the δ(CH) vibrations preferentially create CO stretch ν(CO), and the δ(OH) create methyl rock ρ(CH3) (2−5 ps), and then ρ(CH3) and ν(CO) decay into phonons (5−15 ps). Three stages are also seen in ν(OH) VER except that the first stage involves exciting every other vibration to some extent. About 10% of ν(OH) decay occurs in four stages, where the first stage is ν(OH) → ν(CH). The IR−Raman 3D technique is compared to other multidimensional vibrational spectroscopies. This 3D method is presently the most powerful technique for studying VER in condensed phases. It is sensitive to overtone and combination transitions buried under more intense fundamental absorption spectra. In addition it is sensitive to very weak cascaded anharmonic couplings responsible for the excitation of successive generations of daughter vibrations.