# Femtosecond

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A femtosecond is the SI unit of time equal to 10−15 or 1/1,000,000,000,000,000 of a second; that is, one quadrillionth, or one millionth of one billionth, of a second.[1] For context, a femtosecond is to a second as a second is to about 31.71 million years; a ray of light travels approximately 0.3 µm (micrometers) in 1 femtosecond, a distance comparable to the diameter of a virus.[2]

The word femtosecond is formed by the SI prefix femto and the SI unit second. Its symbol is fs.[3]

A femtosecond is equal to 1000 attoseconds, or 1/1000 picosecond. Because the next higher SI unit is 1000 times larger, times of 10−14 and 10−13 seconds are typically expressed as tens or hundreds of femtoseconds.

Shorter times

• Typical time steps for molecular dynamics simulations are on the order of 1 fs.[4]
• The waves of visible light oscillate with a period (reciprocal frequency) of about 2 femtoseconds ${\displaystyle {\lambda \over {c}}={600\times 10^{-9}~{\rm {m}} \over 3\times 10^{8}~{\rm {m}}~{\rm {s}}^{-1}}=2.0\times 10^{-15}~{\rm {s}}}$. The precise period depends on the energy of the photons, which determines their color. (See wave–particle duality.) This time can be calculated by dividing the wavelength of the light by the speed of light (approximately 3 × 108 m/s) to determine the time required for light to travel that distance.[5]
• 1.3 fs – cycle time for 390-nanometer light, at the transition between violet visible light and ultraviolet[5]
• 2.57 fs – cycle time for 770-nanometer light, at the transition between red visible light and near-infrared[5]
• 15 fs – the swiftest chemical reaction, such as the absorption of a light photon in the Photosynthetic antenna molecule
• 200 fs – the average chemical reaction, such as the reaction of pigments in an eye to light[5]
• 300 fs – the duration of a vibration of the atoms in an iodine molecule[6]

Longer times