# Sublimation rate of molecular crystals - role of internal degrees of freedom

## Abstract

It is a common practice to estimate site desorption rate from crystal surfaces with an Arrhenius expression of the form v{sub eff} exp(-{Delta}E/k{sub B}T), where {Delta}E is an activation barrier to desorb and v{sub eff} is an effective vibrational frequency {approx} 10{sup 12} sec{sup -1}. However, such a formula can lead to several to many orders of magnitude underestimation of sublimation rates in molecular crystals due to internal degrees of freedom. We carry out a quantitative comparison of two energetic molecular crystals with crystals of smaller entities like ice and Argon (solid) and uncover the errors involved as a function of molecule size. In the process, we also develop a formal definition of v{sub eff} and an accurate working expression for equilibrium vapor pressure.

- Authors:

- Publication Date:

- Research Org.:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

- Sponsoring Org.:
- USDOE

- OSTI Identifier:
- 940512

- Report Number(s):
- UCRL-JRNL-227406

TRN: US200824%%75

- DOE Contract Number:
- W-7405-ENG-48

- Resource Type:
- Journal Article

- Resource Relation:
- Journal Name: Journal of Physical Chemistry B, vol. 111, N/A, November 30, 2007, pp. 14290; Journal Volume: 111

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 36 MATERIALS SCIENCE; 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; 37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ARGON; DEGREES OF FREEDOM; DESORPTION; MOLECULAR CRYSTALS; SUBLIMATION; VAPOR PRESSURE

### Citation Formats

```
Maiti, A, Zepeda-Ruiz, L A, Gee, R H, and Burnham, A.
```*Sublimation rate of molecular crystals - role of internal degrees of freedom*. United States: N. p., 2007.
Web.

```
Maiti, A, Zepeda-Ruiz, L A, Gee, R H, & Burnham, A.
```*Sublimation rate of molecular crystals - role of internal degrees of freedom*. United States.

```
Maiti, A, Zepeda-Ruiz, L A, Gee, R H, and Burnham, A. Fri .
"Sublimation rate of molecular crystals - role of internal degrees of freedom". United States.
doi:. https://www.osti.gov/servlets/purl/940512.
```

```
@article{osti_940512,
```

title = {Sublimation rate of molecular crystals - role of internal degrees of freedom},

author = {Maiti, A and Zepeda-Ruiz, L A and Gee, R H and Burnham, A},

abstractNote = {It is a common practice to estimate site desorption rate from crystal surfaces with an Arrhenius expression of the form v{sub eff} exp(-{Delta}E/k{sub B}T), where {Delta}E is an activation barrier to desorb and v{sub eff} is an effective vibrational frequency {approx} 10{sup 12} sec{sup -1}. However, such a formula can lead to several to many orders of magnitude underestimation of sublimation rates in molecular crystals due to internal degrees of freedom. We carry out a quantitative comparison of two energetic molecular crystals with crystals of smaller entities like ice and Argon (solid) and uncover the errors involved as a function of molecule size. In the process, we also develop a formal definition of v{sub eff} and an accurate working expression for equilibrium vapor pressure.},

doi = {},

journal = {Journal of Physical Chemistry B, vol. 111, N/A, November 30, 2007, pp. 14290},

number = ,

volume = 111,

place = {United States},

year = {Fri Jan 19 00:00:00 EST 2007},

month = {Fri Jan 19 00:00:00 EST 2007}

}