Ultra fast laser spectroscopy
The term ultrafast spectroscopy one summarizes spectroscopic measurement methods, their temporal resolution is in the range of femtoseconds. Usually the temporal change of spectral characteristics is pursued, such as the relaxation of an excited state of the chromophore present in the solution in.
Basics
Examples of common means of ultrafast spectroscopy of investigated phenomena:
- Vibrational excitation and energy transfer in molecules ( molecular spectroscopy )
- Course of chemical reactions (eg, in photochemistry )
- Solvation processes in liquids
- Charge carrier transport in semiconductors
These processes typically occur at very short time scales of a few femtoseconds to a few hundreds of picoseconds or a few nanoseconds. A direct detection eg by photodiodes or photomultiplier is due to the limited temporal resolution of electronic devices (at best a few nanoseconds ) is not possible. To observe such phenomena requires a femtosecond laser, which deliver ultra-short light pulses, the time resolution required by optical means. Since light pulses are used as the measuring instrument, therefore, it is all-optical properties which can be observed, especially the transmission, emission or frequency conversion.
Pump-probe experiments
In a pump-probe experiment ( engl. pump-probe experiment ) the system under study by means of a short, intense laser pulse is in an excited electronic state ( excitation ). A second laser pulse, which is compared to the excitation pulse delayed in time (eg by an extended light path, Eng. Delay line ), the response of the system is measured by the time elapsed since the excitation time (query). Then varying the delay time for each delay and measures the current (transient ) response of the system. Plotting the measured values thus obtained against the delay time, then we obtain insight into the dynamics of the running after excitation processes.
Single - and multi-channel detection
Optical responses are observed either at a single (single channel detection) or at multiple wavelengths of a spectrum (multi - or multi-channel detection). Multichannel detection of the response in different spectral ranges can be either serial accomplished by varying the wavelength of a relatively narrow band probe pulse, or by parallel spectrally resolved detection of a broadband probe pulse (white light ). As femtosecond lasers generally produce light pulses in a small spectral region needs to query the pulse frequency-converted by nonlinear optical processes and are converted into white light. The spectral response at a fixed delay time is referred to as transient spectrum.
Measurable sizes
Transmission change
In the study of the transmission characteristics of a fuel system considering the change in transmission which is caused by an excitation pulse. In general, the measured value is indicated as a change in optical density.
Three effects change the transmission behavior compared to the non - excited state:
- Ground state bleaching, Eng. ground state bleaching ( GSB): The intense excitation pulse has placed in the ideal case, all molecules in the system in an excited state. Thus, the ground state is drained, so it can no longer absorb. It is said that the ground state has been bleached.
- Stimulated emission, Eng. stimulated emission (SE): an electronic level was populated by the excitation pulse, which relaxes by fluorescence to the ground state. The intensity of the excitation has the effect that this level is besetzungsinvertiert, so that the interrogation pulse generating stimulated emission.
- Absorption of the excited state, Eng. excited state absorption (ESA ) has the populierte by the excitation excited state, as well as the ground state, a characteristic absorption spectrum, which is dominated by the excitation with respect to the absorption of the ( bleached ) ground state.
All three processes with characteristic decay rate constants.
Emission
Due to the excitation pulse states can be excited in the system under investigation, which relax radiant in the further course. The case emitted in all directions in space light provides information on the fluorescence lifetime and the energy spacing of the states involved.
Frequency conversion
A particularly sensitive to interfacial test method is the sum frequency spectroscopy, in which the frequency of the laser pulses used are converted into a non-linear process at a boundary surface.
- A spectroscopic method