M. C. Nuss

Femtosecond spectroscopy of the first events of the photochemical cycle in bacteriorhodopsin

The first steps in the photochemistry of bacteriorhodopsin (BR) are investigated with light pulses of 160 fs duration. Four samples are studied: (i) the purple membrane, (ii) deuterated purple membrane, (iii) BR trimers and (iv) BR monomers. In all samples the first intermediate J is formed within 430±50 fs. No isotope effect is observed in the formation of J upon deuteration, in contrast to previous reports with much higher excitation energies. Thus proton movement to or from the retinal Schiffs base is not relevant during the first step. Comparing the data for trimeric and monomeric BR suggests an upper limit of 50 fs for the transfer of excitation energy from the excitonically coupled trimer to a single retinal chromophore.

Femtosecond carrier relaxation in semiconductor‐doped glasses

Light induced changes of absorption in semiconductor‐doped glasses are studied on the femtosecond and picosecond time scale. Extremely rapid absorption recovery times of 200 fs are found when carriers are excited with large excess energy (500 meV) above the absorption edge of the semiconductor‐doped glasses. Excitation close to the band edge results in a slower absorption recovery with a time constant of τ≂100 ps.

A new Raman technique of superior spectral resolution

Raman-active vibrational modes are coherently excited by the transient stimulated Raman process. A subsequent delayed probe of relatively long duration interacts with the freely relaxing vibrations. Raman spectra are generated with higher resolution and more accurate peak positions than in conventional Raman spectroscopy. In liquid cyclohexane four new Raman lines were readily detected in the frequency range 2870–2920 cm−1.

Stable operation of a synchronously pumped colliding-pulse mode-locked ring dye laser.

Pulses of 100-fsec duration are obtained by synchronous pumping of a colliding-pulse ring dye laser with a mode-locked Ar(+)-ion laser. Stable operation of the synchronously pumped colliding-pulse mode-locked laser over hours was obtained by a suitable choice of the distance between the gain and the absorber in combination with an appropriate pump-pulse sequence.

Narrowing of spectral lines beyond the natural or dephasing line width

Transient excitation and gated or delayed observation provides a narrowing of the inherent line width of the transition. Possible experimental one- and two-photon systems are discussed and the loss of signal with line narrowing is calculated. The general case of lines broadened by dephasing processes is treated. Contact is made with the line narrowing in recent Raman type experiments.

Prolonged-excitation coherent Raman spectroscopy with spectral resolution beyond the transition linewidth using two tunable picosecond dye lasers

A time-resolved coherent anti-Stokes Raman technique is demonstrated that yields a spectral resolution beyond the linewidth obtained in spontaneous Raman spectroscopy. Two picosecond dye lasers, independently tunable with low timing jitter, are used. The coherent material excitation is generated by long pump pulses and monitored by short delayed probing pulses. Transiently narrowed Raman spectra are recorded by scanning the lasers over the resonances of interest and simultaneously monitoring the coherent scattering signal as a function of the excitation frequency. The technique is applied to molecular vibrations in liquids. The differences between the technique presented and standard coherent anti-Stokes Raman spectroscopy are demonstrated.

A Picosecond Raman Technique with Resolution Four Times Better than Obtained by Spontaneous Raman Spectroscopy

A new Raman technique is presented which allows to observe a Raman transition with a bandwidth smaller than the common spontaneous Raman line-width.

Transient Coherent Raman Spectroscopy: Two Novel Ways of Line Narrowing

Recently, a new Raman technique was introduced, which allows to improve the spectral resolution beyond the limitations of spontaneous Raman spectroscopy /1/. Here we present the first data of the short excitation and prolonged interrogation technique using a continuously tunable excitation from a cw mode-locked dye laser system. In addition, we show a related but different line narrowing method, which uses prolonged excitation and delayed probing /2/. The smooth tunability in both techniques allows the quantitative determination of frequency positions and amplitudes of individual transitions. In congested spectral regions new information, not available from spontaneous Raman data, is obtained.