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1: Fast Processes

In physics, chemistry and biology, a lot of effort and research is directed to understand the dynamics of various processes. To put it simply, people like to watch system parameters changing over time. Depending on the size of the object being watched, the time scales on which the changes take place may vary from very slow to extremely fast (from human perspective). We can try to list several of such processes and arrange them in a table (see also Fig. 1):



Typical duration


Evolution of species

0.4 million years (1.2×1013 s)

homo rhodesiensis to homo sapiens

Population formation

100 years (3×109 s)

Takes forest to grow

Animal lifetime span


65.4 years (2×109 s)

40 days (3×106 s)

A statistical Lithuanian[1]


Animal movement

1 s

0.01 s

Hand gesture

Hummingbird wing flap

Biochemical reaction

20 s

0.01 s



10-3 s

10-5 s

Protein synthesis

Complex enzymatic reaction (e.g., ATP synthase produces ATP molecule from ADP and phosphate)

Action potential change in a neuron

Signalling state formation in a bacterial photoreceptor PYP (photoactive yellow protein)

Carotenoid triplet state lifetime in a photosynthetic antenna complex

Elementary biophysical processes

10-10 s


10-12 s


10-13 s


Full charge separation in a photosynthetic reaction centre

Excitation energy transfer from photosynthetic antenna to the reaction centre.

Retinal isomerization in bacteriorhodopsin and sensory rhodopsin (vision).


Fig. i) Different natural processes, their timescales and instruments for following them.


It is obvious that biological processes cover the timescales spread over at least 26 orders of magnitude. This dynamic range is enormous – no single instrument can cover it. Therefore, different tools are required in order to follow different processes. The slowest tool we have is probably a calendar; more scientifically oriented of us will prefer radioactive dating methods. They follow the processes stretched over centuries and millennia. Further down, on the time scales of human lives, one would probably use a newspaper or a chronicle to describe them. Even faster events are recorded using fast film cameras or photographic cameras with short exposure times (useful, for example, for sport events or monitoring how a lion is chasing a gazelle, timescales down to 0.001 s). Electronic devices provide access to the realm of microseconds, nanoseconds, down to hundreds of picoseconds. Even faster processes, the durations of which are tens of picoseconds and less, require the fastest tool available in nature – light itself. Light (and other electromagnetic waves) travel in vacuum at the constant speed of roughly 3×108 m/s. To visualize it better, let’s look at another table with familiar distances and the time intervals it takes light to travel them:


1 s

300 000 km

Mileage of an average car from manufacturing to recycling

1 ms

300 km

Similar to distance from London to Paris (344 km)

1 ms

300 m

Roughly perimeter of a football field

1 ns

30 cm

Large male foot

1 ps

0.3 mm

Thickness of the beer can walls

1 fs

0.3 mm

Thickness of the rainbow-colored oil film on a puddle


Comparison of the two tables immediately shows that some biological processes happen so fast that even the fastest thing known (light) manages to cover a distance of some microns during the entire event. Let us designate (somewhat arbitrarily) all the processes happening faster than within 1 ms ultrafast processes. The area of science that explores ultrafast processes in atoms, molecules, crystals and glasses using light-based spectral techniques is called time-resolved (or ultrafast) spectroscopy.

Ultrafast phenomena are tough to handle even using light, fast as it may be. Therefore, in order to investigate them, light needs to be controlled in an especially precise manner. The light sources providing unprecedented control of light parameters are lasers. They have been invented in 1960, and have since reached perfection allowing the scientist to control fully such light properties as

Color (wavelength)
Duration of pulse (flash)

It is this unprecedented degree of control over light, laser spectroscopy is a gold mine in investigating ultrafast phenomena. Further we will discuss the general principles of the lasers used for time-resolved spectroscopy; after that, we will describe several most popular time-resolved spectroscopic techniques used in investigating ultrafast phenomena in biology and chemical physics.


[1] 2005 data, life expectancy for Lithuanian males