Vibrational cooling of a molecule

In summary, at supersonic temperatures, rotational levels thermalize quickly, but vibrational levels take longer to settle.
  • #1
Malamala
299
26
Hello! I understand that many of the non-optical methods used to cool down degrees of freedom in a molecule (e.g. buffer gas cooling, supersonic expansion) are able to cool down translational and rotational, but not vibrational motion. Is this because the gap between vibrational levels is much higher, so, for example in a buffer gas, one would need to get rid of all that energy with just one collision, which is highly unlikely? However, mathematically, shouldn't we still expect a Boltzman distribution of vibrational levels, too, regardless of their spacing (i.e. they should thermalize with the buffer gas)? Can someone help me understand this issue (if it is indeed an issue) with vibrational cooling? Thank you!
 
Physics news on Phys.org
  • #2
With hot molecules, molecule-molecule collisions turn translational energy into vibrational energy by exciting the molecules into higher vibrational states. This results in a Boltzmann distribution for vibrational state occupancy. If you suddenly cool the translational degrees of freedom, then there will be no source of vibrational excitation. Excited vibrational levels of the ground electronic state decay to lower vibrational states due to electric quadrupole-allowed spontaneous emission on a timescale of 10-100ms lifetime (pretty darn slow). This is why translational cooling =/= vibrational cooling. It takes time for the vibrational levels to settle into a Boltzmann distribution, often longer than you can keep your species trapped.
 
  • Informative
  • Like
Likes Malamala and berkeman
  • #3
Twigg said:
With hot molecules, molecule-molecule collisions turn translational energy into vibrational energy by exciting the molecules into higher vibrational states. This results in a Boltzmann distribution for vibrational state occupancy. If you suddenly cool the translational degrees of freedom, then there will be no source of vibrational excitation. Excited vibrational levels of the ground electronic state decay to lower vibrational states due to electric quadrupole-allowed spontaneous emission on a timescale of 10-100ms lifetime (pretty darn slow). This is why translational cooling =/= vibrational cooling. It takes time for the vibrational levels to settle into a Boltzmann distribution, often longer than you can keep your species trapped.
Thank you! How about rotational levels? If I am to buffer gas cool the molecules, will I have the same problem i.e. the rotational motion will not be Boltzman distributed at the same temperature as the translation motion and it will take a long time (probably even longer than vibrational case, as the lifetimes are longer) for the rotational motion to reach thermal equilibrium?

And why does supersonic expansion lead to rotational cooling, but not vibrational?
 
  • #4
Sorry for messy reply. Let me know if anything unclear. Energy splitting is much lower for rotational levels, so its easier for collisions to cause diabatic excitations between rotational states, allowing thermalization. Even at supersonic cool temperatures rotational states thermalize quickly.

Regarding my last post, in hindsight I'm actually not sure what the dominant cooling rate for vibrations is (collisions or spontaneous emission) at supersonic beam temperatures. But spontaneous emission sets the lower limit for how fast the excited states come down.

The advantage of a CBGB here is the lower velocity, meaning for a given beam length you get more time to thermalize (experiment size is finite in real world applications).

Even if you do end up with a high rotational temperature, rotational repumping is easy peasy compared to vibrational repumping, though both take a lot of precious time (which translates into beam length if you don't have the ability to trap your molecules).
 

1. What is vibrational cooling of a molecule?

Vibrational cooling is a process in which the excess energy of a molecule's vibrational modes is dissipated, resulting in a decrease in the molecule's temperature. This is achieved through collisions with a cold buffer gas or by radiative cooling.

2. Why is vibrational cooling important?

Vibrational cooling is important because it allows for the study and manipulation of molecules at low temperatures, which is essential for many scientific and technological applications. It also plays a crucial role in understanding chemical reactions and molecular dynamics.

3. How does vibrational cooling work?

Vibrational cooling works by transferring the excess energy of a molecule's vibrational modes to a cold buffer gas through collisions. This causes the molecule to lose energy and decrease in temperature. Alternatively, radiative cooling can also be used, where the molecule releases excess energy in the form of photons.

4. What are the benefits of using vibrational cooling?

Using vibrational cooling allows for the study of molecules at low temperatures, which can reveal important information about their structure and behavior. It also enables the manipulation and control of molecules, which is useful for applications such as quantum computing and precision measurement.

5. What are some challenges in achieving vibrational cooling?

One of the main challenges in achieving vibrational cooling is finding an efficient and effective way to transfer the excess energy of the molecule's vibrational modes to the buffer gas. This requires careful selection of the buffer gas and optimization of experimental parameters. Another challenge is maintaining a low enough temperature for a sufficient amount of time to observe the effects of vibrational cooling.

Similar threads

  • Atomic and Condensed Matter
Replies
4
Views
1K
  • Atomic and Condensed Matter
Replies
2
Views
3K
  • Atomic and Condensed Matter
Replies
3
Views
3K
Replies
2
Views
643
  • Other Physics Topics
Replies
6
Views
3K
  • High Energy, Nuclear, Particle Physics
Replies
13
Views
914
  • Introductory Physics Homework Help
Replies
2
Views
2K
  • Quantum Physics
Replies
2
Views
2K
  • DIY Projects
Replies
23
Views
4K
Replies
1
Views
1K
Back
Top