Time-resolved spectroscopy
In physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 and physical chemistry
Physical chemistry
Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...

, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

 techniques. Most often, processes are studied that occur after illumination of a material, but in principle, the technique can be applied to any process that leads to a change in properties of a material. With the help of pulsed laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

s, it is possible to study processes that occur on time scales as short as 10−14 seconds. The rest of the article discusses different types of time-resolved spectroscopy.

Transient-absorption spectroscopy

Transient-absorption spectroscopy is an extension of absorption spectroscopy. Here, the absorbance at a particular wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 or range of wavelengths of a sample is measured as a function of time after excitation
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....

 by a flash of light. In a typical experiment, both the light for excitation ('pump') and the light for measuring the absorbance ('probe') are generated by a pulsed laser. If the process under study is slow, then the time resolution can be obtained with a continuous (i.e., not pulsed) probe beam and repeated conventional spectrophotometric
In chemistry, spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength...


Examples of processes that can be studied:
  • Optical gain spectroscopy of semiconductor laser materials.
  • Chemical reactions that are initiated by light (or 'photoinduced chemical reactions');
  • The transfer of excitation energy between molecules, parts of molecules, or molecules and their environment;
  • The behaviour of electrons that are freed from a molecule or crystalline material.

Other multiple-pulse techniques

Transient spectroscopy as discussed above is a technique that involves two pulses. There are many more techniques that employ two or more pulses, such as:
  • Photon echoes.
  • Four-wave mixing
    Four-wave mixing
    Four-wave mixing is an intermodulation phenomenon in optical systems, whereby interactions between 3 wavelengths produce a 4th wavelength in the signal. It is similar to the third-order intercept point in electrical systems...

     (involves three laser pulses)

The interpretation of experimental data from these techniques is usually much more complicated than in transient-absorption spectroscopy.

Nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...

 and electron spin resonance are often implemented with multiple-pulse techniques, though with radio waves and micro waves instead of visible light.

Time-resolved infrared spectroscopy

Time-resolved infrared (TRIR) spectroscopy also employs a two-pulse, "pump-probe" methodology. The pump pulse is typically in the UV region and is often generated by a high-powered Nd:YAG laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

, whereas the probe beam is in the infrared region. This technique currently operates down to the picosecond time regime and surpasses transient absorption and emission spectroscopy by providing structural information on the excited-state kinetics of both dark and emissive states.

Time-resolved fluorescence spectroscopy

Time-resolved fluorescence spectroscopy is an extension of fluorescence spectroscopy
Fluorescence spectroscopy
Fluorescence spectroscopy aka fluorometry or spectrofluorometry, is a type of electromagnetic spectroscopy which analyzes fluorescence from a sample. It involves using a beam of light, usually ultraviolet light, that excites the electrons in molecules of certain compounds and causes them to emit...

. Here, the fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...

 of a sample is monitored as a function of time after excitation by a flash of light. The time resolution can be obtained in a number of ways, depending on the required sensitivity and time resolution:
  • With fast-detection electronics (nanoseconds and slower)
  • With Time Correlated Single Photon Counting, TCSPC (picoseconds and slower)
  • With a streak camera
    Streak camera
    A streak camera is an instrument for measuring the variation in a pulse of light's intensity with time. They are used to measure the pulse duration of some ultrafast laser systems, and for applications such as time-resolved spectroscopy and LIDAR....

     (picoseconds and slower)
  • With optical gating (femtoseconds-nanoseconds) - a short laser pulse acts as a gate
    A gate is a point of entry to a space enclosed by walls, or a moderately sized opening in a fence. Gates may prevent or control entry or exit, or they may be merely decorative. Other terms for gate include yett and port...

     for the detection of fluorescence light; only fluorescence light that arrives at the detector at the same time as the gate pulse is detected. This technique has the best time resolution, but the efficiency is rather low. An extension of this optical gating technique is to use a "Kerr gate"
    Kerr effect
    The Kerr effect, also called the quadratic electro-optic effect , is a change in the refractive index of a material in response to an applied electric field. The Kerr effect is distinct from the Pockels effect in that the induced index change is directly proportional to the square of the electric...

    , which allows the scattered Raman
    Raman spectroscopy
    Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...

    signal to be collected before the (slower) fluorescence signal overwhelms it. This technique can greatly improve the signal:noise ratio of Raman spectra.
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