Transient Hole-burning Experiments Research Articles

The Role of the Solvent in the Condensed-Phase Dynamics and Identity of Chemical Bonds: The Case of the Sodium Dimer Cation in THF.

When a solute molecule is placed in solution, is it acceptable to presume that its electronic structure is essentially the same as that in the gas phase? In this paper, we address this question from a simulation perspective for the case of the sodium dimer cation (Na2+) molecule in both liquid Ar and liquid tetrahydrofuran (THF). In previous work, we showed that, when local specific interactions between a solute and solvent are energetically on the order of a hydrogen bond, the solvent can become part of the chemical identity of the solute. Here, using mixed quantum/classical molecular dynamics simulations, we see that, for the Na2+ molecule, solute-solvent interactions lead to two stable, chemically distinct coordination states (Na(THF)4-Na(THF)5+ and Na(THF)5-Na(THF)5+) that are not only stable themselves as gas-phase molecules but that also have a completely new electronic structure with important implications for the excited-state photodissociation of this molecule in the condensed phase. Furthermore, we show through a set of comparative classical simulations that treating the solute's bonding electron explicitly quantum mechanically is necessary to understand both the ground-state dynamics and chemical identity of this simple diatomic molecule; even use of the quantum-derived potential of mean force is insufficient to describe the behavior of the molecule classically. Finally, we calculate the results of a proposed transient hole-burning experiment that could be used to spectroscopically disentangle the presence of the different coordination states.

Rapid switching between slow and fast light by frequency-modulated transient spectral hole-burning

The generation of slow and fast light in the same transient hole-burning experiment by two laser-frequency modulation methods is reported. The schemes allow for rapid switching between slow and fast light on very short time scales. Experiments were carried out in the R1(±3/2) line of a pale green (0.04 wt. % Cr) emerald, Be3Al2Si6O18:Cr3+ in π polarization (E∥c) at 2.3 K in a low magnetic field B∥c of 12 mT. Maximum delays and advancements for a 115 ns Gaussian pulse were 65 and −25 ns, respectively, corresponding to group indices of 7.8·103 and −3·103. The present schemes point to possible applications in all-optical switches.

Revisiting the pump–probe polarized transient hole-burning of the hydrated electron: Is its absorption spectrum inhomogeneously broadened?

Although simulations have predicted that the hydrated electron’s absorption spectrum is inhomogeneously broadened, the experiments in the Literature that test this prediction have produced contradictory results. In this Letter, we describe new polarized transient hole-burning experiments on solvated electrons in both water and methanol. Although we chose excitation wavelengths that should have maximized any polarization dependence in the bleaching dynamics, we were unable to observe any anisotropy. We conclude that the absorption spectrum of the hydrated electron is more homogeneously broadened than suggested by the simulations, although the extent to which the band is inhomogeneously broadened remains an open question.

Effects of low magnetic fields in transient spectral hole-burning of the R 1-line in emerald, Be 3Al 2Si 6O 18:Cr(III)

Polarized transient spectral hole-burning in the R 1-line ( 2 A ̄ ( 2E) ← 4A 2) of Chatham lab grown emerald is reported as a function of low magnetic fields B∥ c and B⊥ c (<16 mT) and temperature. The experiments yield g-factors g ⊥( 4A 2)=1.96 ± 0.02, g ∥( 4A 2)=1.97 ± 0.02 and g ∥[ 2 A ̄ ( 2E)]=1.02 ± 0.03. The hole patterns indicate that spin-lattice relaxation between the Zeeman split spin levels of the ground and excited state is slow at 2.5 K. Spin-lattice relaxation by an Orbach process appears to become efficient for the 2 A ̄ ( 2E) levels at temperatures >8 K on the 2.5 ms experimental timescale but thermalization between the ±3/2 4A 2 levels stays relatively slow up to 25 K. The potential of transient hole-burning experiments is manifested by the present experiments: g-factors and spin-lattice relaxation times can be measured simultaneously for the ground and the excited state.

Solvent effects on the ultrafast dynamics and spectroscopy of the charge-transfer-to-solvent reaction of sodide

In “outer sphere” electron transfer reactions, motions of the solvent molecules surrounding the donor and acceptor govern the dynamics of charge flow. Are the relevant solvent motions determined simply by bulk solvent properties such as dielectric constant or viscosity? Or are molecular details, such as the local solvent structure around the donor and acceptor, necessary to understand how solvent motions control charge transfer? In this paper, we address these questions by using ultrafast spectroscopy to study a photoinduced electron transfer reaction with only electronic degrees of freedom: the charge-transfer-to-solvent (CTTS) reaction of Na− (sodide). Photoexcitation of Na− places the excited CTTS electron into a solvent-bound excited state; motions of the surrounding solvent molecules in response to this excitation ultimately lead to detachment of the electron. The detached electron can then localize either in an “immediate” contact pair (in the same cavity as the Na atom), which undergoes back electron transfer to regenerate Na− in ∼1 ps, or in a “solvent-separated” contact pair (one solvent shell away from the Na atom), which undergoes back electron transfer in tens to hundreds of picoseconds. We present detailed results for the dynamics of each step of this reaction in several solvents: the ethers tetrahydrofuran, diethyl ether and tetrahydropyran and the amine solvent hexamethylphosphoramide (HMPA). The results are interpreted in terms of a kinetic model that both incorporates spectral shifting of the reaction intermediates due to solvation dynamics and accounts for anisotropic spectral diffusion in polarized transient hole-burning experiments. We find that the rate of CTTS detachment does not correlate simply with any bulk solvent properties, but instead appears to depend on the details of how the solvent packs around the solute. In contrast, the rate for back electron transfer of solvent-separated contact pairs varies inversely with solvent polarity, indicating a barrier to recombination and suggesting that this reaction lies in the Marcus inverted regime. For immediate contact pairs, the rate of recombination varies directly with solvent polarity in the ethers but is slowest in the highly polar solvent HMPA, suggesting that the spatial extent of the solvated electron in each solvent is one of the major factors determining the recombination dynamics. The fact that each step in the reaction varies with solvent in a different way implies that there is not a single set of solvent motions or spectral density that can be used to model all aspects of electron transfer. In addition, all of the results and conclusions in this paper are compared in detail to related work on this system by Ruhman and co-workers; in particular, we assign a fast decay seen in the near-IR to solvation of the CTTS p-to-p excited-state absorption, and polarization differences observed at visible probe wavelengths to anisotropic bleaching of the Na− CTTS ground state.

Picosecond ir hole-burning spectroscopy on HDO iceIh

Transient hole-burning experiments using picosecond ir pulses in the region of the OH stretching vibration of crystalline HDO (in ${\mathrm{D}}_{2}\mathrm{O}$) ice are reported; holes with a minimum width of $26{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are measured, proving the OH band to be inhomogeneously broadened. The inhomogeneous distribution having a half width of approximately $25{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ can be related to structural disorder in ice. A vibrational lifetime of $0.5\mathrm{ps}&lt;~{T}_{1}&lt;~2\mathrm{ps}$ is found for the OH stretching mode. Additional features in the transient data are attributed to the dissipation of excess energy; particularly microscopic energy flow in the immediate surroundings of primarily excited OH groups can be monitored.

Permanent and microsecond transient hole-burning in free-base tetraphenylporphin using a quantum-well diode laser

Permanent as well as transient spectral holes in the S 1 ← S 0 absorption of tetraphenylporphin in polymeric hosts have been burned and detected at λ = 637 nm with a time resolution in the microsecond range using a single-mode quantum-well semiconductor diode laser. Whereas in polystyrene no measurable spectral diffusion occurs, the holes broaden by about a factor of two in polymethylmethacrylate between 30 μs and 300 s. In a transient hole-burning experiment on the triplet-triplet excited state transition at 780 nm no holes were observed within a scan range of 10 GHz and with delays t d as short as 3 μs.

Temperature and solvent viscosity dependence of conformational dynamics of Zn-substituted myoglobin observed by nanosecond time-resolved transient hole-burning spectroscopy

Conformational fluctuations of a protein myoglobin in solution have been investigated by a time-resolved transient hole-burning experiment with a temporal resolution of ~ 10 ns over the temperature range of 180–300 K. The temperature dependence of the observed relaxation time τ r has been well fitted by the Vogel-Fulcher law. The solvent viscosity dependence of τ r has been also studied around room temperature.

Excitation Wavelength Dependence of Bacterial Reaction Center Photochemistry. 2. Low-Temperature Measurements and Spectroscopy of Charge Separation

Excitation with spectrally narrow (5 nm), temporally short duration (200 fs) laser pulses at a variety of wavelengths between 848 and 903 nm results in substantial excitation wavelength dependent differences in the evolution of the bacterial reaction center absorbance spectrum both before and after charge separation occurs. The transient holes in the initial electron donor band showed a more resolved vibrational band structure at 20 K, when compared to those of earlier room temperature transient hole-burning experiments (Peloquin, J. M.; Lin, S.; Taguchi, A. K. W.; Woodbury, N. W. J. Phys. Chem. 1995, 99, 1349). The dominant vibrational band observed is at 120 cm-1, in agreement with dynamic measurements of coherent oscillations on this time scale (Vos, M. H.; Rappaport, F.; Lambry, J.-C.; Breton, J.; Martin, J.-L. Nature 1993, 363, 320). At both room temperature and low temperature, there is a distribution of P to P* transition energies due to a distribution of the protein conformations in the ground state. At 20 K, one can also see a distribution of P* to P stimulated emission energies. As might be expected, the barriers to conformational interconversion are more easily crossed at room temperature, resulting in a smaller difference between the mean transition energies of the photoselectable subpopulations on the picosecond time scale relative to those at low temperature. At 20 K, much of this conformational interconversion is apparently lost when exciting near the 0−0 transition wavelength of P. More conformational interconversion appears to take place at low temperature when higher energy excitation is used, implying that P* is vibrationally hot for at least hundreds of femtoseconds following excitation on the blue side of its QY band. Of particular interest is the insensitivity of the overall charge separation kinetics to the selection of ground state conformational subpopulations by different excitation wavelengths. There appears to be very little coupling between the nuclear motion that defines the ground state transition distribution and the charge separation reaction itself. Given the apparently slow rate of vibrational relaxation of some of the excited state modes most strongly coupled to the P to P* transition, the lack of excitation wavelength dependence of the charge separation rate also suggests that these modes are not strongly coupled to the charge separation reaction. What the ground state (and possibly excited state) conformational distributions and interconversions do affect is the kinetic complexity of the absorbance changes in the 800 nm region. Specifically, the extent of involvement of fast, multiexponential decay components in this region depends strongly on the wavelength of excitation.

Stark and Zeeman effects in the lowest excited states of [Zn(bpy) 3](ClO 4) 2:Ru(II). Localized triplet metal-to-ligand charge transfer transitions

Polarized absorption spectra of the metal-to-ligand charge transfer (MLCT) transitions in the title system are reported. Strong linear dichroisms are observed in accord with the crystal structure which shows that only two of the bpy ligands are crystallographically equivalent. The MLCT transitions to the third ligand are found to lie several hundred wavenumbers higher in energy. A strong angular dependence of the Zeeman effect in luminescence is observed when varying the direction of the magnetic field in the metal-ligand plane. These patterns are quantitatively analyzed in terms of localized transitions. The three lowest-excited electronic states are well described as the components of an orbitally non-degenerate spin triplet state with a very large zero field splitting. All three lowest-energy origins show the same large splitting in an applied electric field, confirming their charge-transfer nature. Luminescence and excitation line narrowing experiments establish that the splitting is a pseudo-Stark effect. Stark-swept transient hole-burning experiments provide a homogeneous line width of ≈ 15 MHz for the origin II in [Zn(bpy) 3](CIO 4) 2:Ru(II) at 1.8 K.

Stark-swept transient hole burning and resonant luminescence line narrowing in [Zn(2,2′-bipyridine) 3](ClO 4) 2:Ru(II)

Resonant luminescence line narrowing and Stark-swept transient hole-burning experiments were performed in the electronic origins of the lowest excited states of the title material. The energies of the two lowest excited states are well correlated. No fine structure was observed in the range 300 MHz–30 GHz for the resonantly and non-resonantly narrowed origin II and I, respectively. This is at variance with the interpretation of ODMR experiments. At 4.2 K an upper limit of ≈ 25 MHz is deduced for the homogeneous line width of origin II. A large pseudo-Stark splitting of the origins is observed corresponding to |Δμ| ≈ 5.5 D. This value is used to calibrate Stark-swept transient hole-burning experiments which needed ± 33 V/cm to measure the entire hole shape. A homogeneous line width of 14 ± 2 MHz is found for origin II at 1.5 K. The large Stark effect is the first direct evidence for the charge-transfer character of the lowest excited states in [Ru(bpy) 3] 2+. The results are consistent with the localized nature of the lowest excited states.

Photoreactive Disordered Systems Studied by Permanent and Transient Hole-Burning

Abstract Permanent and transient hole-burning experiments carried out on various types of amorphous systems at temperatures between 0.3 and 20 K are presented. Optical dephasing on organic molecules in glasses and polymers measured on different time scales are discussed in relation to spectral diffusion processes and compared to the literature. The pure dephasing time appears to be proportional to the excited state lifetime of the guest. Organic molecules adsorbed on the surface of porous silica glass show that their homogeneous linewidth is an order of magnitude larger than in polymers and glasses in the bulk, but follows the same temperature dependence. Hole-burning results on bacteriochlorophyll in various disordered hosts are compared to those on a isolated light-harvesting photosynthetic pigment-protein complex (B800–850). From the holewidth in the latter the energy transfer rate between aggregated bacteriochlorophyll pigments is determined and compared to time-resolved spectroscopy results in the li...

Discussion of a ‘‘coherent artifact’’ in four-wave mixing experiments

In this paper, we discuss the nonlinear optical effects that arise when stochastic light waves, with different correlation times, interfere in an absorbing medium. It is shown that four-wave mixing signals are generated in several directions that spectrally track the incoming light fields. This effect is particularly relevant to transient hole-burning experiments, where one of these signals could easily be misinterpreted as a genuine hole-burning feature.

Optical dynamics of the reaction center of photosystem II. A hole-burning and photon-echo study

Photon-echo and transient hole-burning experiments on the P-band of the reaction center of photosystem II are reported which show that the P-band exhibits a large homogeneous width. A photon-echo signal with a decay time of 500 ps is also observed in the same spectral region and assigned to extraneous chlorophyll. These observations are discussed with reference to the bacterial reaction center and photosystem I.