Dorte Madsen

TWO-PHOTON DISSOCIATION AND IONIZATION OF LIQUID WATER STUDIED BY FEMTOSECOND TRANSIENT ABSORPTION SPECTROSCOPY

The photodissociation and photoionization of liquid water following two-photon absorption at 266 nm is studied in the spectral range from 213 to 1108 nm with subpicosecond time resolution. Probing in the UV enables the first direct simultaneous observation of the photoproducts eaq−, Haq, and OHaq. This makes it possible to follow the geminate recombination kinetics between the photoproducts and to determine the relative yields of the dissociation and ionization channels. The concentration of hydrated electrons deduced from the visible and near-infrared transient absorption measurements decays by 40%±2% within the first 90 ps due to recombination with OHaq and H3O+. Analyzing our measurements of the hydrated electron concentration using the independent reaction time approximation results in the relative yields of 82%±3% and 18%±3% for recombination with OHaq and with H3O+, respectively. This is in excellent agreement with the relative yield of 82%±10% for recombination with OHaq determined directly from ou...

Temperature dependent relaxation and recombination dynamics of the hydrated electron

The ultrafast solvation and recombination dynamics of the hydrated electron generated by two-photon ionization of water at 4.65 eV is studied by transient absorption spectroscopy as a function of temperature in the range from 277 K to 355 K. The part of the spectral blue shift which is observed in the absorption spectrum of the hydrated electron after 1 ps is purely continuous and is accurately described by the well known analytical expression for the temperature dependent absorption spectrum of the ground state hydrated electron. This indicates that thermal relaxation or more likely solvation of the hydrated electron predominantly causes the blue shift. The survival probability of the hydrated electron shows a strong temperature dependence, which is satisfactory explained by the temperature dependent mobility and reaction rates of the species involved in the recombination. This implies that the average initial separation between the hydrated electron and the ionization site of 〈r0〉=1.0±0.1 nm does not de...

Coherent vibrational ground-state dynamics of an intramolecular hydrogen bond

Abstract We present direct evidence for coherent vibrational motions of low frequency in the v =0 ground state of the high-frequency O–D stretching mode of an intramolecular hydrogen bond. Femtosecond mid-infrared pump–probe studies of hydroxy-deuterated 2-(2 ′ -hydroxyphenyl)benzothiazole in toluene reveal an oscillatory change of O–D stretching mode absorption persisting for several picoseconds after excitation. An infrared resonantly enhanced stimulated Raman process within the bandwidth of the pump pulse creates a vibrational wave packet which is made up of quanta of a 118 cm −1 mode modulating the length of the hydrogen bond and thus the O–D stretching absorption band.

The photoisomerization of aqueous ICN studied by subpicosecond transient absorption spectroscopy

The photolysis of aqueous ICN is studied by transient absorption spectroscopy covering the spectral range from 227 to 714 nm with 0.5 ps time resolution. The experimental data show that when ICN(aq) is photolyzed at 266 nm, it dissociates into I and CN and both the I(2P3/2) and I(2P1/2) channels are populated. Approximately half the fragments escape the solvent cage while the remainder recombines within the solvent cage during the first picosecond. The majority of the recombinations form ICN while only a minor fraction produces the metastable INC isomer. INC and ICN relax to the vibrational ground state within 1 ps in good agreement with theoretical estimates based on the golden rule formalism as well as molecular dynamics simulations. Diffusive recombination involving fragments that have escaped the solvent cage further reduces the quantum yield of I and CN to 10% during the following 100 ps. This recombination produces exclusively ICN.

Femtosecond mid-infrared photon echo study of an intramolecular hydrogen bond

Abstract The vibrational dephasing dynamics of O–H stretching excitations in a sterically well-defined intramolecular hydrogen bond (H bond) is studied by femtosecond photon echo spectroscopy in the mid-infrared. The O–H stretching vibration of phthalic acid monomethyl ester in inert solution displays a fast decay of phase coherence on a sub-100 fs time scale. Anharmonic coupling of the stretching motion to an underdamped low-frequency mode of the H bond gives rise to oscillations in the transient grating decay and the 3-pulse photon echo peak shift. Spectral diffusion dynamics on longer time scales is absent in such intramolecular H bonds, as opposed to the weaker intermolecular H bonds in water.

Femtosecond photolysis of aqueous HOCl

This paper reports an experimental study of the photolysis of aqueous HOCl using femtosecond pulses at 266 nm. The formation of photoproducts is monitored by transient absorption spectroscopy from 230 to 400 nm. The HOCl molecules dissociate with unity quantum yield to form OH+Cl faster than 1 ps, and as a result of the potential along the HO–Cl reaction coordinate, all excess energy is given to the fragments as translational energy. After dissociation, and solvent cage escape, the majority of the Cl and OH fragments recombine after diffusion on a time scale of 50 ps. The diffusion dynamics is studied using a simple model for diffusive recombination and a more extensive molecular dynamics simulation. A minor fraction of the Cl atoms (∼10%) reacts with HOCl in a diffusion limited reaction to form Cl2+OH.

Femtosecond spectroscopy of the dissociation and geminate recombination of aqueous CS2

The photolysis of aqueous CS2 has been studied using subpicosecond transient absorption spectroscopy. CS2 was photolyzed at 6.2 eV and the fast formation and decay of the photoproducts were monitored from 6.2 to 3.96 eV. Upon excitation, aqueous CS2 dissociates into CS+S. However, 93%±2% of the fragments geminately recombine on the electronic ground state potential of CS2 within a few picosecond leaving only 7%±2% of the CS+S fragments separated 100 ps after the excitation. In the gas phase, most of the dissociation occurs on a triplet-state potential surface, and the high recombination yield observed in aqueous solution therefore indicates a strong, solvent-assisted coupling between this state and the singlet ground state of CS2. The vibrationally excited CS2 molecule formed by the recombination transfers its high excess energy to the surrounding water molecules in two processes with time constants 8.4±1 and 33±7 ps. The rotational reorientation time of ground state CS2 is 6±1 ps, suggesting a surprising...

Ultrafast dynamics of liquid water

Using THz-time domain spectroscopy, we have studied the dynamics of pure water in the far-infrared spectral range, where the diffusive and inertial rotational motion of the water molecules is observed. With femtosecond transient absorption spectroscopy we have studied the relaxation (vibration, solvation, thermalization, and translation) of different solutes in liquid water. The solutes include, the hydrated electron, ClO/sub 2/, ClOH, Cl, and CS/sub 2/. Performing the femtosecond experiments at different temperatures gives insight into the connection between the dynamics observed in the far-infrared and the relaxation dynamics of the different solutes. In addition we also discuss the validity of employing macroscopic parameters (diffusion coefficients and temperature) in the description of single molecule dynamics on an ultrashort time scale.

Ultrafast coherent response of hydrogen bonds

Femtosecond pump-probe spectroscopy in the mid-infrared demonstrates the coherent oscillatory modulation of the hydrogen bond distance and its strength in an intramolecular hydrogen bond, giving evidence of anharmonic coupling between high-and low-frequency modes.

Femtosecond spectroscopy of the dissociation and geminate recombination of aqueous carbondisulfide

Summary form only given. Ultrafast spectroscopy on small molecules in solution offers a unique possibility for studying solvent-solute interaction, since the spectroscopic properties of the species involved often are well determined. In this context, photodissociation of tri-atomic molecules stands out as being particularly interesting, since the dissociation of these species may follow several different solvent dependent reaction paths, while still being simple enough to allow for high level calculations of the reaction dynamics. Photodissociation and subsequent recombination have been studied in a number of triatomic molecules in solution including HgI/sub 2/, I/sub 3//sup -/, O/sub 3//sup -/, and ClO/sub 2/. In these systems the photodissociation either leads to a vibrationally excited diatomic fragment which relaxes prior to the recombination, or a fast geminate recombination of the photofragments followed by vibrational relaxation. Apart from the important information about the primary quantum yields of these processes, ultrafast spectroscopy of these molecules have already revealed detailed information about solvent-solute interaction.