Dissociation in diatomic molecules

[kelly0303]

Hello! Let's assume we have a molecule in the ground electronic and vibrational state. Let's assume that the first excited electronic state is very far away so it can be neglected for the purpose of the questions. Also we can assume that the shape of the potential is well-behaved i.e. it is a Morse potential for example with a dissociation energy $D_0$ (this is basically $D_e-\omega/2$ i.e. it ignores the zero point energy). If I want to dissociate the molecule, would any laser with a frequency bigger than $D_0$ work (I assume you would need quite a lot of power as this is non-resonant)? I know that this is what it is commonly done to ionize atoms and molecules (you just send a laser with a power above the ionization threshold), but is it just like that in the dissociation case, too? Also, is $D_0$ a hard limit on dissociation i.e. does the molecule still have a probability of being dissociated even if the laser frequency is below $D_0$ (e.g. it reaches one of the highest vibrational levels), or does it actually have to be above $D_0$. I would appreciate if someone can point me towards some readings about this (ideally something that can help me calculate some dissociation probability for a given molecule). Thank you!

 

[eljose79]

Hello! Thank you for your question. I can provide some insight into your inquiries about dissociation using lasers.

Firstly, in order to dissociate a molecule using a laser, the laser must have a frequency that matches the energy gap between the ground and excited electronic states. This is known as resonant excitation. If the laser frequency is too low, it will not have enough energy to excite the molecule to the desired electronic state and therefore will not cause dissociation.

In the case of a well-behaved potential, such as a Morse potential, the dissociation energy ($D_0$) is a measure of the energy required to break the bond between the atoms in the molecule. Therefore, any laser with a frequency higher than $D_0$ would have enough energy to dissociate the molecule. However, as you mentioned, this would require a significant amount of power as it is non-resonant excitation.

In terms of whether $D_0$ is a hard limit on dissociation, it is possible for a molecule to dissociate even if the laser frequency is below $D_0$. This could occur if the molecule is initially in a highly excited vibrational state, known as a Franck-Condon state, which can have a higher energy than the ground electronic state. In this case, the laser may have enough energy to excite the molecule to the excited electronic state, causing it to dissociate.

As for calculating the dissociation probability for a given molecule, this can be done using quantum mechanical models and simulations. There are also experimental techniques, such as pump-probe spectroscopy, that can measure the dissociation probability under different laser conditions. I would suggest looking into papers and articles on these techniques for further information.

I hope this helps answer your questions. Best of luck with your research!