Multiquantum Excitation Research Articles

Vibrationally Hot Reactants in a Plasmon-Assisted Chemical Reaction.

Recent studies on plasmon-assisted chemical reactions postulate that the hot electrons of plasmon-excited nanostructures may induce a non-thermal vibrational activation of metal-bound reactants. However, the postulate has not been fully validated at the level of molecular quantum states. We directly and quantitatively prove that such activation occurs on plasmon-excited nanostructures: The anti-Stokes Raman spectra of reactants undergoing a plasmon-assisted reaction reveal that a particular vibrational mode of the reactant is selectively excited, such that the reactants possess >10 times more energy in the mode than is expected from the fully thermalized molecules at the given local temperature. Furthermore, a significant portion (∼20%) of the excited reactant is in vibrational overtone states with energies exceeding 0.5 eV. Such mode-selective multi-quantum excitation could be fully modeled by the resonant electron-molecule scattering theory. Such observations suggest that the vibrationally hot reactants are created by non-thermal hot electrons, not by thermally heated electrons or phonons of metals. The result validates the mechanism of plasmon-assisted chemical reactions and further offers a new method to explore the vibrational reaction control on metal surfaces.

Application of multi-step excitation schemes for detection of actinides and lanthanides in solutions by laser spectroscopy

Data on chemiluminescence of chemiluminogen (luminol, 5-amino-1,2,3,4-tetrahydro-1,4-phthalazindione) in aqueous solutions containing Sm(III), U(IV), and Pu(IV) are presented. Chemiluminescence was induced by multiquantum excitation of lanthanide and actinide ions in the range of 4f- and 5f-electron transitions by the schemes two step–one color and two step–two color with the use of dye lasers. A multi-step scheme of chemiluminescence excitation provides highly sensitive and highly selective procedure of detection of lanthanides and actinides in solutions.

Nuances of multi-quantum excitation in solid state NMR of quadrupolar nuclei

In this article, a simple heuristic approach that is in agreement with the density operator formalism is presented for understanding the mechanism of multi-quantum (MQ) excitation in half-integral quadrupolar systems. Employing the population level diagram of the nuclear spin states, the effect of radio-frequency (RF) pulses on multi-level systems is described. Based on this approach, alternate schemes that involve non-equilibrium spin states are proposed for improving the sensitivity of MQ experiments in spin 3/2 and 5/2 systems. Additionally, the utility of frequency sweep techniques in the excitation of MQ transitions is discussed. The results obtained from the analytic theory are substantiated through numerical simulations.

Initiation of luminol chemiluminescence by two-quantum excitation of U(IV) in aqueous solution

Results of experiments on initiation of luminol chemiluminescence by multiquantum excitation of U(IV) in aqueous solution by dye laser radiation in the visible range are reported.

Plasmon single- and multi-quantum excitation in free metal clusters as seen by photoelectron spectroscopy

Plasmons are investigated in free nanoscale Na, Mg, and K metal clusters using synchrotron radiation-based x-ray photoelectron spectroscopy. The core levels for which the response from bulk and surface atoms can be resolved are probed over an extended binding energy range to include the plasmon loss features. In all species the features due to fundamental plasmons are identified, and in Na and K also those due to either the first order plasmon overtones or sequential plasmon excitation are observed. These features are discussed in view of earlier results for planar macroscopic samples and free clusters of the same materials.

Selective energy and phase transfer in the photodissociation of I 2 in argon clusters: Quantum dynamics simulations

Photodissociation of I 2 in the I 2Ar 17 cluster is studied by quantum wavepacket simulations, using time-dependent mean-field potentials. It is found that: (a) Initial energy transfer from the I 2 is mostly to three cluster modes, which undergo multiquantum excitation. Several other modes undergo single-quantum excitation. (b) Coherent phase transfer from the I 2 to specific cluster modes is found. (c) Energy transfer from initially-excited modes to the other modes becomes large after 0.6 ps. Phase space interpretation is provided by computed Wigner distributions in time.

Detection of high-energy adsorbate vibrational modes by atom-surface scattering

It is suggested that high-frequency vibrational modes of surface adsorbates can produce very characteristic signature features in the observable inelastic intensity in experiments using atomic beams as scattering probes. These features consist of peaks in the inelastic background, due to multiquantum excitation of the high-energy vibrational modes, and these peaks are significantly broadened and shifted in the direction of energy loss by the multiphonon scattering arising from the substrate and other low-energy adsorbate modes. Calculations for the scattering of rare gases from several types of adsorbate indicate that, by selecting the probe projectile species and tuning the incident energy and incident beam angle, selected high-energy modes can be excited, and the method can be used to accurately measure the vibrational frequencies and polarizations of these modes.

Energy transfer between collagen-dye molecules as a probe of higher-lying electronic states following multiquantum excitation

Under high-intensity nanosecond 1064 nm irradiation of a collagen-rhodamine 6G (R6G) system, multiquantum excitation of the R6G molecules to their S2 state was facilitated. The observation of this effect was achieved through associated energy transfer to the resonant collagen excited state band. Analogous cascaded energy transfer phenomena, resulting from the S2 electronic state of R6G, were observed in an R6G-collagen-coumarin system.

Synchronized double rotation 2D NMR

Analytical expressions for multi-quantum signal generation of quadrupole nuclei have been derived. Combined with numerical simulation of the double rotor motion, a strategy is suggested for partial sideband suppression in multi-quantum NMR spectra. Synchronization of multi-quantum excitation and selective flip pulses with outer rotor motion increase outer rotor speed effectively two times. This is also demonstrated experimentally by triple–single quantum correlation spectra of 23Na and 67Rb.

Laser multiphoton processes in thin films: Non-linear photochemistry of organosilane polymers

Irradiation of organosilane polymers in the ultraviolet leads to photodegradation of the chain via a bond scission process. Here photodegradation by visible laser multi-quantum excitation is reported. The process is characterized by a sharp threshold at power densities of approximately 2.5 GW/cm 2. The photochemistry of the system beyond this power density proceeds non-linearly leading to vaporization of the absorbing film and 100% bleaching of the ultraviolet absorption bands. Utilization of laser wavelengths whose two-photon energies are insufficient to access the ultraviolet system, produced no measurable photodecomposition.

Interaction of atomic and molecular systems with high-intensity ultraviolet radiation

The interaction of atomic and molecular species with picosecond ArF* laser radiation is studied at intensities up to 1015 W/cm2. Anomalously strong, collision-free multiple ionization is observed. Standard theoretical models of stepwise ionization fail to describe the results. The experimental findings point to a collective response of the atom. At intensities of ~1013 W/cm2, selective multiquantum excitation of autoionizing states in Kr, followed by stimulated emission at wavelengths as short as 91.6 and 93 nm, is observed. The 93-nm radiation is tunable over a 600-cm−1 interval, whereas the 91.6-nm frequency is fixed. It appears that electron collisions redistribute energy among excited states.

Role of coherence in multiquantum excitation of molecules by IR laser radiation

Role of coherence in multiquantum excitation of molecules by IR laser radiation

Phase Coherence in Multiple Pulse Optical Spectroscopy

In this paper we describe a new technique for the generation of multiple pulse phase coherent sequences in optical spectroscopy. The technique is an extension of the acousto-optic modulation and fluorescence detection methods developed for optical transitions by Zewail and Orlowski (Zewail et al., Chem. Phys. Lett.48, 256 (1977); Orlowski et al., Chem. Phys. Lett.54, 197 (1978)). Application of these multiple pulse trains (of different phases) to optical transitions of two-level and multilevel systems is demonstrated experimentally. It is shown that they can be used to (i) suppress spontaneous emission background, (ii) enhance coherent transients such as photon echoes, (iii) measure additional relaxation parameters in systems with complex rotational-vibrational levels, and (iv) enhance the effective laser bandwidths through composite pulse trains, as demonstrated on I2 gas. Finally, the potential of this development is extended to the possibility of observing selective multiquantum excitation in molecules.

Translation to vibration energy transfer in O + NH3 and O + CO2 collisions

Vibrational excitation of NH3 and CO2 in collision with atomic oxygen is investigated for energies of 1–10 eV. A classical hard-sphere model used previously for diatomic molecules is adopted. Four ``diatomic'' models of NH3, in which the H3 complex assumes the role of a single atom, are used to study excitation of the ν2 mode. The results for the four models are very similar despite the model differences. It is shown that excitation of the ν1 and ν2 modes of CO2 can be computed very accurately through construction of the appropriate diatomic molecules. The classical energy transfers for CO2 and NH3 are converted to excitation probabilities of specific quantum states by using the assumption that the time-dependent interaction causing the excitation is a driving force independent of the oscillator coordinate. It is found that multiquantum excitation is important, the more so at higher energies. A scaling technique is used to estimate energy transfer for a more realistic atom-atom interaction. This results in greatly reduced multiquantum excitation for energies less than 5 eV. Excitation of the stiffer modes is also investigated. It is found that while excitation of the NH3ν4 mode is comparable to that of the ν2 mode at higher energies, the other modes may be neglected for the entire energy range studied.

Divalent Rare Earth Spectra Selection Rules and Spectroscopy of SrCl2:Sm2+

Selection rules for vibronic transitions appearing in $4f\ensuremath{\rightarrow}4f$ fluorescence of divalent rare earth (R${\mathrm{E}}^{2+}$) ions in alkaline-earth halide lattices are derived. These are applied to the cases of Sr${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$, Sr${\mathrm{Cl}}_{2}$: ${\mathrm{Sm}}^{2+}$, and Ca${\mathrm{F}}_{2}$: ${\mathrm{Tm}}^{2+}$. On the basis of a simple theory which is outlined, selection rules for nonradiative (relaxation) transitions of R${\mathrm{E}}^{2+}$ ions in $m3m$ symmetry are also presented. From these rules one can predict instances where spectral broadening of fluorescent lines should occur due to spontaneous emission of phonons, with consequent lifetime broadening. Comparison with experiment seems to verify these predictions. The structure of the $4{f}^{n\ensuremath{-}1}5d$ bands of R${\mathrm{E}}^{2+}$ ions is shown to be due to vibronic coupling of pure electronic states with states involving multiquantum excitation of even-parity vibrations. Simple coupling schemes for the $4{f}^{n\ensuremath{-}1}5d$ levels are examined. Results of measurements of Sr${\mathrm{Cl}}_{2}$: ${\mathrm{Sm}}^{2+}$ absorption and fluorescence spectra are presented. Lifetimes and quantum efficiencies are discussed.