FEMTOCHEMISTRY (Preparation of coherent state superpositions)

[altered-gravity]

Greetings,

I´m taking contact with femtochemistry. As you know, the goal of this area is the real time following of the molecular dinamics, providing the first real experimental data from transition states, resonances, mechanisms of energy redistribution.. and so on.

The key to study theese time-resolved dinamics is the preparation of time dependent coherent superposition of quantum states, generating a wave packet and studying it´s evolution. Ultrashort pulses are able to do this.

In pump-probe configuration, the pump pulse sets the zero-time of reaction and induces the coherent state, then the probe pulse gets info about it at different time delays.

Can anyone talk to me about it?
Why are ultrashort pulses capable of doing this?
What is the spectroscopic mechanism of probing theese wave packets?

Anything you know will be helpful for me. Thanks!

 

[Mike H]

I think you pretty much are on to one of your answers - the reason femtosecond laser pulses are capable of probing such fast physical and chemical processes is because they do occur over very short timescales, and one needs to be able to sample data points at a fast enough rate to get an accurate picture of the system as it evolves. It's the same underlying idea as monitoring a reaction that takes an hour to finish with, let's say, a UV/Vis spectrophotometer that takes a spectrum every minute. In the case of the pump-probe experiment, obviously, the reaction is initiated by the pump laser instead of mixing two reagents together.

I'm not entirely sure to what you're exactly referring to when you ask what is the spectroscopic mechanism of probing these wavepackets. In terms of the optics, it can vary depending on the problem and one's budget. In my old lab (where we did pump-probe experiments, but not at the femtosecond timescale), we had a pump laser pulse coming in perpendicular to our continuous wave probe laser beam, and we measured the transient absorbance. Some optics benches have it so that a single pulse is split and then one half is sent to the sample as the pump pulse and then the delayed probe pulse follows it in after being redirected through the desired optics maze. In terms of what's being measured, that too can vary. One can measure transient absorbance and decay, fluorescence...I would presume there would be variations as well as a few others, but am not current on the latest tricks of the trade.

In terms of how are the femtosecond pulses are generated, I suspect something like Demtroder's Laser Spectroscopy might be a better source than me, since I would just end up running to my copy to answer any such questions. :)

 

[altered-gravity]

Thanks Mike H

Just now I´ve found Zewail´s nobel lecture about femtochemistry, if you´re interested have a look at this link
http://www.nobel.se/chemistry/laureates/1999/zewail-lecture.pdf