I know it must be less than 1 ps, but anyone have a more precise knowledge about it?
For a direct photodissociation without any fancy processes, a very crude rule of thumb would be the typical vibration times, at least if you only care about the order of magnitude. I think, 1 ps is a pretty generous upper limit.
For anything more precise or special, you would need to specify the laser pulse (wave length especially), molecule, and a precise definition of what you mean with "time scale of photodissociation".
I think, it is dependent to laser intensities and energy bound of molecule. For example for direct photo dissociation of C-H bond , it is needed to least 20fs to start dissociation process.
different molecules may need different time.For example ,acetylene(C2H2),it is about 10~20fs.For Br2，it is about 150fs.You can see this article:Visualizing electron rearrangement in space and time during the transition from a molecule to atoms.
and the following url.
http://science.sciencemag.org/content/354/6310/308.long
To Lorentz:
Thanks for your response, which I really appreciate!
The typical vibration time of molecule is about 100 fs, according to A. Zewail's study on molecular Iodine. So the time scale of direct dissociation of molecule would be the same scale with the typical vibration time ?
The term "time scale of photodissocation" means how long it takes to break up the bond by photons.
To Elnaz:
Thanks for your response!
I understand that it depends on the laser intensity, photon energy, and bond strength. So for the C-H bond, what do you mean that "start dissociation"? And how do you get this "20 fs"? by experiment or by calculation? If you know relating reference, please tell me.
I think this reference is a good reference about this field.
Dissociation of Methane in Intense Laser Fields
Sufan Wang , Xiaoping Tang , Lirong Gao , Mohamed E. Elshakre ,† and Fanao Kong *
Institute of Chemistry, Chinese Academy of Science, Beijing 100080, China
J. Phys. Chem. A, 2003, 107 (32), pp 6123–6129
@Pengji: Yes.
Allow me to elaborate a bit more. There are roughly two major factors regarding the time scales:
1. The (reduced) mass of the molecule. Everything goes slower with heavier molecules, and from classical mechanics you should get a scaling factor of sqrt(mu) here.
2. The forces acting on the molecule during the dissociation.
Under the crude assumption that the repulsive and the binding forces are similar in magnitude, you get my rule of thumb above. However, these assumptions are only good enough if you only need an approximate number without spending days on it.
If you _do_ care about the difference between, say, 50 fs and 200 fs, you have to do more theory; in particular, you need the potential for the dissociating state. Also, you need to define what it means for the molecule to be dissociated. When the atoms are 5 Angstroem distant? 10? Obviously, this changes your numbers somewhat, and it depends on your specific use case.
Since both vibrations and dissociation are dictated by nucleonic movememt, the ps/sub-ps vibrational time-scale are a general top limit. Indeed the shape of the dissociative potential upon which it occurs will effect the time (slope of potential->force ->acceleration), though different excitations to different curves (linear? non-linear?) may result in different kinetic-energy-release (KER), affecting the dissociation time. e.g. for displacement of 1 angstrom with 1 eV KER and mu=1 amu -> 7fs, and scale by sqrt(mu) for higher mass. Of course, one must define dissociation, also dependent on the potential, for instance - at a displacement where most of the energy is released. For an experimental work on an anionic di-atom you can see DOI: 10.1021/acs.jpca.6b13008 (hot from the "oven", just accepted)
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