Double Resonance Excitation Research Articles

Room-Temperature Single-Molecule Infrared Imaging and Spectroscopy through Bond-Selective Fluorescence.

Infrared (IR) spectroscopy stands as a workhorse for exploring bond vibrations, offering a wealth of chemical insights across diverse frontiers. With increasing focus on the regime of single molecules, obtaining IR-sensitive information from individual molecules at room temperature would provide essential information about unknown molecular properties. Here, we leverage bond-selective fluorescence microscopy, facilitated by narrowband picosecond mid-IR and near-IR double-resonance excitation, for high-throughput mid-IR structural probing of single molecules. We robustly capture single-molecule images and analyze the combined polarization dependence, vibrational peaks, linewidths, and lifetimes of probe molecules with representative scaffolds. From bulk to single molecules, we find that vibrational lifetimes remain consistent, while linewidths are narrowed by approximately twofold and anisotropy becomes more pronounced. Additionally, unexpected peak shifts from single molecules were observed, attributed to the generation of new modes due to previously unexplored dimerization, supported by quantum chemistry calculations. These findings underscore the importance of infrared analysis on individual single molecules in ambient environments, offering molecular information crucial for functional imaging and the investigation of the fundamental properties and utilities of luminescent molecules.

Research on double resonant excitation in a triangular electrode ion trap with asymmetric geometry.

The triangular electrode linear ion trap with asymmetric geometry has been reported to possess a high ion unidirectional ejection efficiency and a reasonable mass resolution. To further improve its performance, a double resonant excitation method involving a dipolar and a quadrupolar resonant excitation was applied here. The dipolar excitation method was carried out by applying a supplementary alternating voltage out of phase to one pair of the electrodes, whereas the quadrupolar excitation (QE) method was carried out by adding a supplementary alternating voltage in phase to another pair of electrodes. Numerical simulations were performed to explore the impact of the frequency difference between the alternating current (AC) and the QE voltage (∆ω), the frequency of the AC voltage (ωAC), and the QE voltage amplitude (VQE). The mass resolution could be improved to ~4700 , which was approximately twice compared to that with only dipolar resonant excitation, and the ion unidirectional ejection efficiency could be improved to 97%. Even with a high scan rate of 6000 Da/s, there was minimal loss of mass resolution caused by increased scan rate in double resonant excitation mode. By employing the double resonant excitation method, the mass resolution could be further increased while maintaining a considerably high ion unidirectional ejection efficiency, which might be a simple and practical approach for developing a high-performance miniature ion trap mass analyzer.

Efficient transfer of population between rotational levels in deuterated ammonia using THz transitions

We demonstrate the efficient transfer of population between rotational levels of ND 3 using a commercially available table-top THz source. We use a Stark decelerator to prepare a packet of para- ND 3 molecules in the 1 1 − upper inversion component of the rotational ground state, and induce the 2 1 + ← 1 1 − electric dipole allowed transition both in the presence of an electric field and under field free conditions. Using THz chirps to address all hyperfine components, transfer efficiencies up to 87% are reached, limited by the power of the THz source. Population transfer from the 1 1 − to the 2 1 − level is demonstrated either by driving directly the 2 1 − ← 1 1 − transition in a field, or by a 1 1 − → 1 1 + → 2 1 − double-resonance excitation scheme. In addition, we present a novel method to produce packets of ND 3 ( 1 1 − ) with pure M J = 0 or | M J | = 1 character using the Stark decelerator, and a method to probe the M J composition of a packet of molecules using THz excitation.

Ion-core switching in Rydberg series of XeKr

We investigate XeKr in high Rydberg states by mass-resolved optical-optical double resonance excitation spectroscopy. We excite XeKr via two-photon absorption to the v* = 0 and v* = 2 vibrational levels of an intermediate electronically excited state of Xe*Kr correlated to Xe*6p[5/2]2 and excite Xe*Kr further via one-photon absorption to the high Rydberg states of Xe**Kr in the energy range of 93200–97400 cm−1. The potential energy curves and the quantum defects of the d-Rydberg and s-Rydberg series of Xe**Kr, converging the A 2Π3/2 state of XeKr+ are determined. From the kinetic energy release in the predissociation process, we conclude that the ion-core switching occurs by the interaction between the bound electronic states of Xe**Kr converging to the A 2Π3/2 state of XeKr+ and the repulsive electronic states of low-lying electronic states of XeKr**. Based on numerical simulations, we interpret the dependence of the yield of the ion-core switching on the excitation energy in terms of the population transfer from the bound electronic states of the high Rydberg states Xe**Kr, converging to the A 2Π3/2 state of XeKr+, to those converging to the electronic ground X2 Σ 1 / 2 + state of XeKr + .

Simulation of double resonant excitation of ions in an asymmetric linear ion trap mass analyzer.

Improving the analytical performance of linear ion traps (LITs) is crucial for the advancement of high-performance LIT mass spectrometers. In this study, a double resonant excitation method was employed in an asymmetric LIT to achieve high ion unidirectional ejection efficiency and enhanced mass resolution. The asymmetric trapping field was generated by stretching one x electrode with a distance α. The double resonant excitation was achieved by applying an alternating voltage out of phase and a supplementary alternating voltage in phase to the x and y electrode pairs of the LIT, respectively. Numerical simulations of ion trajectories were performed to validate the effectiveness of this method. The mass resolution of the asymmetric LIT with double resonant excitation could be improved to ~3800, which was over two times compared to that with only dipolar resonant excitation, while both reached ~90% in ion unidirectional ejection efficiency. By employing the double resonant excitation method, the mass resolution could be improved significantly in the asymmetric LIT, while maintaining a considerably high ion unidirectional ejection efficiency. This method might provide a general solution for enhancing ion detection efficiency and mass resolution of LIT mass spectrometers.

Mm-wave Rydberg-Rydberg transitions gauge intermolecular coupling in a molecular ultracold plasma.

Out-of-equilibrium, strong correlation in a many-body system can trigger emergent properties that act to constrain the natural dissipation of energy and matter. Signs of such self-organization appear in the avalanche, bifurcation, and quench of a state-selected Rydberg gas of nitric oxide to form an ultracold, strongly correlated ultracold plasma. Work reported here focuses on the initial stages of avalanche and quench and uses the mm-wave spectroscopy of an embedded quantum probe to characterize the intermolecular interaction dynamics associated with the evolution to plasma. Double-resonance excitation prepares a Rydberg gas of nitric oxide composed of a single selected state of principal quantum number, n0. Penning ionization, followed by an avalanche of electron-Rydberg collisions, forms a plasma of NO+ ions and weakly bound electrons, in which a residual population of n0 Rydberg molecules evolves to a state of high orbital angular momentum, ℓ. Predissociation depletes the plasma of low-ℓ molecules. Relaxation ceases and n0ℓ(2) molecules with ℓ ≥ 4 persist for very long times. At short times, varying excitation spectra of mm-wave Rydberg-Rydberg transitions mark the rate of electron-collisional ℓ-mixing. Deep depletion resonances that persist for long times signal energy redistribution in the basis of central-field Rydberg states. The widths and asymmetries of Fano line shapes witness the degree to which coupling in the arrested bath (i) broadens the allowed transition and (ii) mixes the local network of levels in the ensemble.

Optical–optical double resonance study of the H' 1 (1D2) ion-pair state of I35Cl: Rovibrational structure and electronic transition dipole moment function

The H' 1 (1D2) ion-pair state of I35Cl, which correlates to the I+ (1D2) + Cl− (1S0) dissociation limit, was analyzed spectroscopically by H' 1 (1D2) ← A3Π1 ← X1Σ+ optical–optical double resonance excitation. In total, 751 transitions covering 0 ≤ vH' ≤ 20 and 10 ≤ JH' ≤ 41 were observed. An e/f-parity splitting (Ω-type doubling) in the H' 1 (1D2) state was observed. By fitting 656 transitions to Dunham type expansion, Dunham coefficients for the H' 1 (1D2) state were determined to be Y00 = 51615.194(14), Y10 = 184.2440(59), Y20 = −0.63083(67), Y30 = 0.001400(21), Y01 = 0.05780468(82), Y11 = −2.2465(19) × 10−4, and Y21 = −2.227(88) × 10−7 cm−1. The vibration-dependent Ω-type doubling constants were determined to be q0 = 1.97(17) × 10−5, q1 = 8.76(38) × 10−6, and q2 = −3.498(18) × 10−7 cm−1. The electronic transition dipole moment function of the H' 1 (1D2) – A3Π1 transition in the range of 2.977–3.683 Å was determined experimentally by analyzing the dispersed ultraviolet (UV) fluorescence spectra combined with the fluorescence lifetime of the H' 1 (1D2) state.

Selective photo-excitation of molecules enabled by stimulated Raman pre-excitation.

Double resonance excitation, where the energies of vibrational and electronic molecular transitions are combined in a single, sequential excitation process, was introduced in the 1970s but has only been recently applied to microscopy due to the immense progress in Raman spectroscopy. The value of the technique is in combining the chemical selectivity of IR or Raman excitation with the much larger cross-sections of electronic transitions. Recently, it has been shown to be particularly suited for the detection and identification of chromophores at low concentrations and in the presence of spectral crosstalk. However, despite its low quantum yield per pulse sequence, we believe the technique has potential for selective photochemical transformations. There are some cases (e.g., the selective excitation of optogenetic switches) where the low yield may be overcome by repeated excitations to build up biochemically relevant concentrations. Here we show that double resonance excitation using general, non-resonant Raman pre-excitation is a viable candidate for selectively promoting molecules to chemically active energy levels. The use of non-resonant Raman pre-excitation is less constraining than resonant Raman (used in previous double resonance microscopy works) since the choice of Raman pump-Stokes frequencies may be rather freely chosen.

On the double resonance activation of electrostatically actuated microbeam based resonators

Electrostatically actuated microelectromechanical system (MEMS) devices have shown prominent potential in various applications. However, despite their low power consumption, they do require high input voltages to be actuated. This is even worse if these devices are driven around their mechanical resonant state. Assuming a double resonance excitation scheme, both electrically and mechanically, activates the system’s mechanical and electrical resonances simultaneously. This double resonance activation was recently verified experimentally to alleviate the problem of high actuating voltage/displacement near the resonant state. Therefore, in this work, an analytical electro-mechanical coupled model for a double-resonance-driven microbeam, assuming a classical nonlinear beam model combined with an RLC electric circuit model is first proposed and then numerically examined. Good match among the numerical simulations and the experimental data when the electrical resonance frequency band is sufficiently high is demonstrated. The suggested model can be used to further optimize certain MEMS designs and therefore fully benefit from this double resonance activation scheme in improving MEMS sensors and actuators.

Large enhancement of the effective second-order nonlinearity in graphene metasurfaces

Using a powerful homogenization technique, one- and two-dimensional graphene metasurfaces are homogenized both at the fundamental frequency (FF) and second harmonic (SH). In both cases, there is excellent agreement between the predictions of the homogenization method and those based on rigorous numerical solutions of Maxwell equations. The homogenization technique is then employed to demonstrate that, owing to a double-resonant plasmon excitation mechanism that leads to strong, simultaneous field enhancement at the FF and SH, the effective second-order susceptibility of graphene metasurfaces can be enhanced by more than three orders of magnitude as compared to the intrinsic second-order susceptibility of a graphene sheet placed on the same substrate. In addition, we explore the implications of our results on the development of new active nanodevices that incorporate nanopatterned graphene structures.

Voltage and Deflection Amplification via Double Resonance Excitation in a Cantilever Microstructure.

Cantilever electrostatically-actuated resonators show great promise in sensing and actuating applications. However, the electrostatic actuation suffers from high-voltage actuation requirements and high noise low-amplitude signal-outputs which limit its applications. Here, we introduce a mixed-frequency signal for a cantilever-based resonator that triggers its mechanical and electrical resonances simultaneously, to overcome these limitations. A single linear RLC circuit cannot completely capture the response of the resonator under double resonance excitation. Therefore, we develop a coupled mechanical and electrical mathematical linearized model at different operation frequencies and validate this model experimentally. The double-resonance excitation results in a 21 times amplification of the voltage across the resonator and 31 times amplitude amplification over classical excitation schemes. This intensive experimental study showed a great potential of double resonance excitation providing a high amplitude amplification and maintaining the linearity of the system when the parasitic capacitance is maintained low.

Time-resolved double-resonance spectroscopy: Lifetime measurement of the electronic state of molecular sodium.

We report on the lifetime measurement of the state of Na2 molecules, produced in a heat-pipe oven, using a time-resolved spectroscopic technique. The level was populated by two-step two-color double resonance excitation via the intermediate state. The excitation scheme was done using two synchronized pulsed dye lasers pumped by a Nd:YAG laser operating at the second harmonics. The fluorescence emitted upon decay to the final state was measured using a time-correlated photon counting technique, as a function of argon pressure. From this, the radiative lifetime was extracted by extrapolating the plot to collision-free zero pressure. We also report the calculated radiative lifetimes of the ro-vibrational levels in the range of = 0-200 with J = 1 and J = 31 using the LEVEL program for bound-bound and the BCONT program for bound-free transitions. Our calculations reveal the importance of the bound-free transitions on the lifetime calculations and a large difference of about a factor of three between the J = 1 and J = 31 for the = 40 and = 100, respectively, due to the wavefunction alternating between having predominantly inner and outer well amplitude.

Hyperfine coupling of the iodine and β1g ion-pair states

Detailed studies of I2(β1g, vβ = 13, Jβ ∼ vD = 12, JD and D, 48, JD ∼ β, 47, Jβ) rovibronic state coupling have been carried out using two-step two-color, hν1 + hν2 and hν1 + 2hν2, optical–optical double resonance excitation schemes, respectively. The hyperfine interaction satisfying the = 0, 1 selection rules (magnetic-dipole interaction) has been observed. No electric-quadrupole hyperfine coupling ( = 2) has been found. The dependences of ratios of luminescence intensities from the rovibronic states populated due to the hyperfine coupling to those from optically populated ones on energy gaps between these states have been experimentally determined. The matrix elements as well as the hyperfine structure constant have been obtained using these dependences. It is shown that they increase slightly with the vibrational quantum number of the states.

Double resonant excitation of the second harmonic of terahertz raditation in dielectric-graphene layered metamaterials

Excitation of the second harmonic of THz radiation is investigated theoretically in the planar multilayered structure dielectric-graphene-dielectric-graphene–…. It is studied the case of the oblique incidence of the s-polarized fundamental wave, where the electric field is parallel to the interfaces, and generation of the p-type second harmonic wave occurs. The original concept is proposed to employ the double resonance arrangement for the effective generation of the second harmonic. The double resonant case can be realized when a high-permittivity dielectric is at the input of the structure and the vacuum is at the output. The high efficiency is demonstrated; the second harmonic reflectance coefficient is ≥0.01 under realistic values of the collision frequency in graphene >1012 s−1. Such a great efficiency, which is four–five orders of magnitude higher than reported for the graphene-dielectric structures previously, is proposed for the first time. To compute the nonlinear surface currents, two approaches were used, the kinetic and the hydrodynamic. A qualitative agreement between two approaches, proven in the present modeling, ensures an applicability of the results.

Single Analyzer Neutral Loss Scans in a Linear Quadrupole Ion Trap Using Orthogonal Double Resonance Excitation.

In this follow-up paper to our previous work on single analyzer precursor ion scans in a linear quadrupole ion trap (Snyder, D. T.; Cooks, R. G. Single analyzer precursor ion scans in a linear quadrupole ion trap using orthogonal double resonance excitation. J. Am. Soc. Mass Spectrom. 2017, DOI: 10.1007/s13361-017-1707-y), we now report the development of single analyzer neutral loss scans in a linear quadrupole ion trap using orthogonal double resonance excitation. Methodologically, there are three key differences between single analyzer precursor ion scans and neutral loss scans under constant radiofrequency (rf) conditions: (1) in the latter experiment, both excitation and ejection frequencies must be scanned, whereas in the former the ejection frequency is fixed, (2) the need to maintain a constant neutral loss while incrementing both precursor and product ion masses, complicated by the complex relationship between secular frequency and mass, requires use of two simultaneous frequency scans, both linear in mass, and (3) because the ejection frequency is scanned, a third ac signal occurring between the ac excitation and ac ejection frequency scans must also be applied and scanned in order to reject artifact peaks caused by ejection of unfragmented precursor ions. Using this methodology, we demonstrate neutral loss scans on a commercial linear ion trap using mixtures of illicit drugs and acylcarnitines. We also demonstrate neutral loss scanning on a Populus deltoides leaf and on a lignin sample, both significantly more complex mixtures.

Single Analyzer Precursor Ion Scans in a Linear Quadrupole Ion Trap Using Orthogonal Double Resonance Excitation.

Reported herein is a simple method of performing single analyzer precursor ion scans in a linear quadrupole ion trap using orthogonal double resonance excitation. A first supplementary AC signal applied to the y electrodes is scanned through ion secular frequencies in order to mass-selectively excite precursor ions while, simultaneously, a second fixed-frequency AC signal is applied orthogonally on the x electrodes in order to eject product ions of selected mass-to-charge ratios towards the detector. The two AC signals are applied orthogonally so as to preclude the possibility of (1) inadvertently ejecting precursor ions into the detector, which results in artifact peaks, and (2) prevent beat frequencies on the x electrodes from ejecting ions off-resonance. Precursor ion scans are implemented while using the inverse Mathieu q scan for easier mass calibration. The orthogonal double resonance experiment results in single ion trap precursor scans with far less intense artifact peaks than when both AC signals are applied to the same electrodes, paving the way for implementation of neutral loss scanning in single ion trap mass spectrometers. Graphical Abstract ᅟ.

Experimental and theoretical investigation of the vibrational band structure of the 1 Πu5-1 Πg5 high-spin system of C2.

Vibrational levels of the recently observed high-spin transition (1 Πu5-1 Πg5) of dicarbon [P. Bornhauser et al., J. Chem. Phys. 142, 094313 (2015)] are explored by applying non-linear double-resonant four-wave mixing and laser-induced fluorescence methods. The deperturbation of the d Πg3, υ = 8 and 1 Πg5, υ = 3 states results in accurate molecular constants for the υ = 3 "dark" quintet state. In addition, the spin-orbit interaction constant is determined and parameters for the upper Swan level d Πg3, υ = 8 are improved. The first excited vibrational state of 1 Πu5 is observed by performing perturbation-assisted intersystem crossing via "gateway" states in the d Πg3, υ=6∼1 Πg5,υ= 0 system. The rotationally resolved spectra yield 11 transitions to 1 Πu5, υ = 1 that include four spin-substates. Data reduction results in accurate molecular constants of this vibrational level in the shallow potential energy surface of this state. Finally, υ = 1 and 2 of the lower quintet state (1 Πg5) are measured by performing perturbation-assisted double-resonant excitation to the 1 Πu5, υ = 0 state and observing dispersed fluorescence. The obtained molecular constants are compared with high level ab initio computations at the multi-reference configuration interaction (MRCI) level of theory by using a large correlation consistent basis set or, alternatively, by applying the computationally less demanding method of explicitly correlated multi-reference configuration interaction (MRCI-F12). The spectroscopic accuracy of both methods is evaluated by comparison with the experimental findings.

Quantum-state-resolved reactivity of overtone excited CH4 on Ni(111): Comparing experiment and theory.

Quantum state resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH4 on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH4 in three different vibrational symmetry components, A1, E, and F2, of the 2ν3 antisymmetric stretch overtone vibration as well as in the ν1+ν3 symmetric plus antisymmetric C-H stretch combination band of F2 symmetry. The quantum state specific dissociation probability S0 (sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH4 dissociation by Auger electron spectroscopy. We observe strong mode specificity, where S0 for the most reactive state ν1+ν3 is an order of magnitude higher than for the least reactive, more energetic 2ν3-E state. Our first principles quantum scattering calculations show that as molecules in the ν1 state approach the surface, the vibrational amplitude becomes localized on the reacting C-H bond, making them very reactive. This behavior results from the weakening of the reacting C-H bond as the molecule approaches the surface, decoupling its motion from the three non-reacting C-H stretches. Similarly, we find that overtone normal mode states with more ν1 character are more reactive: S0(2ν1) > S0(ν1 + ν3) > S0(2ν3). The 2ν3 eigenstates excited in the experiment can be written as linear combinations of these normal mode states. The highly reactive 2ν1 and ν1 + ν3 normal modes, being of A1 and F2 symmetry, can contribute to the 2ν3-A1 and 2ν3-F2 eigenstates, respectively, boosting their reactivity over the E component, which contains no ν1 character due to symmetry.

Observations and analysis with the spline-based Rydberg–Klein–Rees approach for the 31Σg+ state of Rb2

Ro-vibrational term values of the 3(1)Σg (+) state of (85,85)Rb2 and (85,87)Rb2 and resolved fluorescence spectra to the A(1)Σu (+) state are recorded following optical-optical double resonance excitation. The experimental data are heavily perturbed, and as a result, the standard analysis based on Dunham series representation of the energy levels fails. The analysis is done via modeling the adiabatic potential function with the Rydberg-Klein-Rees potential constructed from the generalized smoothing spline interpolation of the vibrational energies Gv and rotational constants Bv.

Luminescence of ion-pair I2 (D′) state in cryogenic perfluorocarbons and SF6 solids

Luminescence of I2-doped CnF2n+2 (n = 1–4) and SF6 cryogenic solids has been studied using an optical-optical double resonance excitation technique. Experiments have shown that the lowest ion-pair state I2(D′) is stable with respect to chemical reaction and fast predissociation in these hosts. The red shift of I2(D′ → A′) transition energy relative to the gas phase shows no correlation with the host polarizability and is ∼3500 cm−1 for all studied solids. Besides the I2(D′) solvation energy, a part of the shift is due to a repulsive interaction of I2(A′) with the host molecules. In the CF4 and SF6 solids, the I2(D′ → A′) spectrum shows evidence of a coupling between electronic transition in the guest molecule and a vibrational transition in the host molecules. This coupling manifests itself via a satellite of the I2(D′ → A′) band, which is shifted to the red by one quantum energy of the strongest IR-active vibrational mode of the CF4 (SF6) molecule. The lifetime of I2(D′) in studied molecular hosts varies from 2.5 ns in CF4 to 13 ns in SF6 solid.