Ultrafast Chemical Reactions in Shocked Nitromethane Probed with Dynamic Ellipsometry and Transient Absorption Spectroscopy

Abstract

Initiation of the shock driven chemical reactions and detonation of nitromethane (NM) can be sensitized by the addition of a weak base; however, the chemical mechanism by which sensitization occurs remains unclear. We investigated the shock driven chemical reaction in NM and in NM sensitized with diethylenetriamine (DETA), using a sustained 300 ps shock driven by a chirped Ti:sapphire laser. We measured the solutions' visible transient absorption spectra and measured interface particle and shock velocities of the nitromethane solutions using ultrafast dynamic ellipsometry. We found there to be a volume-increasing reaction that takes place around interface particle velocity up = 2.4 km/s and up = 2.2 km/s for neat NM and NM with 5% DETA, respectively. The rate at which transient absorption increases is similar in all mixtures, but with decreasing induction times for solutions with increasing DETA concentrations. This result supports the hypothesis that the chemical reaction mechanisms for shocked NM and NM with DETA are the same. Data from shocked NM are compared to literature experimental and theoretical data.

Shock Initiation Microscopy with High Time and Space Resolution

“…In the past several years, multiple research groups have developed tabletop shock generation systems to facilitate a greater understanding of the many facets of the complex phenomena involved in topics ranging from detonation propagation , to chemical reaction pathways under shock conditions , and hot spot temperature measurements . With the exception of the work of Tappan et al .…”

“…In the past several years, multiple research groups have developed tabletop shock generation systems to facilitate a greater understanding of the many facets of the complex phenomena involved in topics ranging from detonation propagation [6,7], to chemical reaction pathways under shock conditions [8][9][10][11], and hot spot temperature measurements [12,13]. With the exception of the work of Tappan et al on microenergetics, where research is conducted at the limit of failing detonation to gain insight into these fail-ure mechanisms [7,14], the sample dimensions in these experiments are below the dimension where traditional detonations can occur [15,16] because the diameter is below the critical diameter and the run distance is often less than the length of the reaction zone.…”

Interaction between measurement time and observed Hugoniot cusp due to chemical reactions

“…Figure 1 illustrates Hugoniot data for acrylonitrile, phenylacetylene, nitromethane, and carbon disulfide measured with plate impact data [8][9][10][11][12][13][14][15] recorded on nanosecond to microsecond time scales along with laser driven shock data measured on picosecond time scales. 4,5,16 The arrows mark the onset of deviation of the Hugoniot from the universal liquid Hugoniot along the particle velocity axis. Clearly, the shorter time scale measurements allow observation along the unreacted Hugoniot to higher particle velocities.…”

“…[1][2][3][4][5] To simplify sample preparation and avoid effects of microstructure, much of the data acquired has been on shocked organic liquids. We have now accumulated enough data that trends are apparent.…”

Development of a broadband reflectivity diagnostic for laser driven shock compression experiments

“…[19][20][21] Using this broadening technique, a considerable amount of work has been done in the last decade developing transient absorption spectroscopy of laser driven shock compressed samples. [22][23][24][25][26][27] This has provided a great deal of insight into the chemical changes accompanying compression; however, the measurements are limited to a single time point per shot and therefore does not probe time-dependent spectral changes.…”