Ultrafast Spectroscopy

The term ultra-short- term spectroscopy is used to summarize spectroscopic measuring methods whose temporal resolution is in the femtosecond range. The change in spectral properties over time is mostly tracked, e.g. For example, the relaxation of the excited state located chromophore in solution.

Basics

Examples of phenomena often examined by means of ultra-short-term spectroscopy:

Vibrational excitation and energy transfer in molecules ( molecular spectroscopy )
Process of chemical reactions (e.g. in photochemistry )
Solvation processes in liquids
Charge carrier transport in semiconductors

These processes typically occur on very short time scales from a few femtoseconds to a few hundred picoseconds or a few nanoseconds . A direct detection z. B. by photodiodes or photomultiplier is not possible due to the limited temporal resolution of electronic devices (at best a few nanoseconds). To observe such phenomena, a femtosecond laser is required whose ultra-short light pulses provide the required time resolution optically. Since light pulses are used as a measuring instrument, it is only optical properties that can be observed, especially transmission , emission or frequency conversion .

Stimulus-query experiments

In an excitation-query experiment ( English pump-probe experiment ), the system under investigation is put into an excited, electronic state (excitation) by means of a short, intense laser pulse. By means of a second laser pulse which is delayed in time compared to the excitation pulse (e.g. by an extended beam path, English delay line ), the response of the system is measured after the time that has elapsed since the excitation (query). The delay time is now varied and the instantaneous (transient) response of the system is measured for each delay. If the measured values obtained in this way are plotted against the delay time, then an insight into the dynamics of the processes occurring after the excitation is obtained.

Single and multi-channel detection

Optical responses are observed either with a single (single-channel detection) or with several wavelengths of a spectrum (multiple or multi-channel detection). With multi-channel detection, the response in different spectral ranges can be achieved either serially by varying the wavelength of a relatively narrow-band interrogation pulse, or by parallel, spectrally resolved detection of a broad-band interrogation pulse (white light). Since femtosecond lasers generally generate light pulses in a small spectral range, the interrogation pulse must be frequency-converted or converted into white light using non-linear, optical processes . The spectral response with a fixed delay time is called the transient spectrum .

Measurable quantities

Change in transmission

When examining the transmission properties of a material system, one looks at the change in transmission caused by an excitation pulse. As a rule, the measurand is given as a change in optical density . Here is the transmission of the excited sample and the transmission of the non-excited sample. ${\ displaystyle - \ ln (T / T_ {0})}$ ${\ displaystyle \ Delta \ mathrm {OD} = \ lg (T_ {0} / T)}$ ${\ displaystyle T}$ ${\ displaystyle T_ {0}}$

Three effects change the transmission behavior compared to the non-excited state:

Ground state bleaching, English ground state bleaching (GSB): The intense excitation pulse ( English pumping pulses ) (typically a few percent) was added to a portion of the molecules in the focal volume to an excited state (i.e., the proportion of the sample which is penetrated by the focused laser beam..). This means that there are fewer molecules in the ground state than before the excitation. As a result, the sample is optically thinner in the spectral range of the ground state absorption and the interrogation pulse is weakened less strongly, i.e. H. is greater than . Thus, the contribution of ground state bleaching to transient absorption is always negative.

{\ displaystyle T}

{\ displaystyle T_ {0}}

{\ displaystyle \ Delta \ mathrm {OD}}

Stimulated emission , English stimulated emission (SE): An electronic level can be occupied (populated) by the excitation pulse, which relaxes to the ground state through fluorescence. If the fluorescence spectrum falls within the range of the interrogation pulse, the radiating relaxation is triggered. The emitted light contributes to the detected intensity of the interrogation pulse, so that even stimulated emission always makes a negative contribution to transient absorption.

Absorption of the excited state, English excited state absorption (ESA): The excited state populated by the excitation has, like the ground state, a characteristic absorption spectrum. The interrogation pulse is absorbed by the molecules in the corresponding excited state, thus causing a decrease in transmission. The absorption of the excited state consequently always has a positive contribution to the transient absorption.

In the case of photochemical reactions, photoproducts can be formed which, analogous to ESA, also make a positive contribution to transient absorption.

All three processes take place with characteristic rate constants .

emission

The excitation pulse can excite states in the system under investigation, which later relax radiantly. The light emitted in all spatial directions provides information about the fluorescence lifetime and the energy gap between the states involved.

Frequency conversion

An examination method that is particularly sensitive to interfaces is sum frequency spectroscopy , in which the frequency of the laser pulses used is converted in a non-linear process at an interface.

literature

Manfred Hugenschmidt: Laser measurement technology: diagnostics of short-term physics . Springer, Berlin / Heidelberg 2007, ISBN 978-3-540-29920-2 , Chapter 11: Ultrashort pulse measurement technology .