Nonequilibrium Processes During Fe(CO)₅ Pyrolysis in a Shock Wave

Abstract

In this work the processes of nonequilibrium radiation and ionization in the weak shock waves (2 < M < 4) in argon or helium, containing a small admixture (0.1–2%) of Fe(CO)5 are experimentally studied. The spectra- and time-resolved measurements, performed using a ICCD camera (StreakStar II, LaVision GmbH) have shown, that the unresolved radiation spectra are situated in the range 400–700 nm. The maximum of the spectra lies approximately at 615 nm. The radiation appeared immediately at the propagating shock front and lasted about 8–12 μs. The following time-resolved measurements, performed using photomultipliers (at 615 ± 10 nm) and calibrated electric probes have shown intensive peaks of emission and electric current with duration of few μs, that correlates with the characteristic time of active growth of iron clusters. The analysis of the results obtained allowed to conclude that the real mechanism of generation of the observed peaks of radiation and ionization is the instant dissociation of Fe(CO)5 causing an active condensation process of a supersaturated vapor of iron atoms, which results in the formation of excited and ionized iron clusters.

Nonequilibrium radiation and ionization during formation of iron clusters in shock waves

“…Measurements were carried out at a rather low intensity of shock waves (Mach numbers from 2 up to 4) at which the equilibrium temperature T 2 behind the shock wave was less than 1500 K. At these temperatures no equilibrium radiation or ionization of any impurity (including alkali metals) could be expected. Nevertheless, both optical measurements and electric probes have recorded intense peaks after shock wave arrival in the mixtures containing Fe(CO) 5 . In test experiments in pure argon and helium no radiation or electric probe signals were registered.…”

“…Besides that, the time profiles of emission at 615 ± 10 nm were recorded by a photomultiplier (PM) with an interference filter. The details of emission measurements are described in [5].…”

“…One of the prospective ways of iron nanoparticle synthesis is the condensation of supersaturated iron vapour formed during the thermal decomposition of iron carbonyl Fe(CO) 5 [1]. To study this process, shock wave investigations of Fe(CO) 5 pyrolysis are quite informative. The time-resolved measurements of Fe-atom, small Fe-cluster and Fe-nanoparticle concentrations present valuable data about the kinetics of Fe-cluster growth and condensed structure formation [2][3][4].…”

“…However, besides the planned results these measurements have revealed some unexpected phenomena. First, about 20 years ago [2], and then in recent experiments [5] intense peaks of nonequilibrium radiation and ionization in a shock wave front in mixtures of argon and helium containing a small addition of iron carbonyl Fe(CO) 5 were observed. Two different mechanisms for this phenomenon have been discussed-high energy collisions within the zone of translation relaxation and recombinative overexcitation of iron clusters formed as a result of the fast decay of carbonyl molecules behind the shock wave.…”

“…A phenomenological analysis of nonequilibrium radiation and ionization registered in shock-heated mixtures containing iron carbonyl [5] led to the preliminary conclusion that the observed phenomena are also caused by an active condensation of iron atoms and they are not related to the processes within the zone of translational relaxation of the shock wave front. However, the time scale of these processes appears to be so short that it could not be resolved in shock tube experiments.…”