20
2 Instrumentation for Cluster Science
of the cluster, P k /P ∞ . This equation helps explain why a cluster distribution is usually
seen with a normal or an inverse gamma distribution while opposed to a logarithmic
decay (i.e. the monomer is more intense than the dimer which is also more intense
than the trimer, etc.). This equation also indicates interpretion why metals with higher
boiling points tend to form smaller clusters even though under the same experimental
conditions (due to the reduced partial pressure of the monomer, and then Φ k ).
Also Eq. 2.6 stands to reason its time independence: once the cluster has exited
from the source it will no longer grow. The time allowed for cluster formation is
also called “aggregation time” [119] which is dependent on the parameters of the
source and the speed/pressure of the backing gas [120]. In experimental design, the
“aggregation time” can be qualitatively ascertained by the flow rate of the backing
gas and the space immediately between the target material and the exit of the source,
called “waiting room”. A practical means of increasing the “aggregation time” (i.e.,
gas pressures in the “waiting room”) is by simply enlarging the flow of the gas.
Nevertheless, the operation to simply increase the flow of the buffer gas could be
insufficient to adjust the “aggregation time”, as the gas is expanding into vacuum
and therefore small changes in flow will do rare changes to the pressure in the source
[121]. In comparison, adjusting the exit nozzle is a more efficient solution. The
conductance “C” of a tube is inversely proportional to its length “L” and directly
proportional to the fourth power of its diameter “D” according to fluid mechanics (C
∝ L
−1 ; C ∝ D
4 ) [121].
2.3 Cluster Reaction Apparatus
The investigations into cluster reactivity have been accomplished by several
approaches [122–145], for example, through the study of the ensuing dynamics of
product evolution of excited species formed on the excitation of the neutral cluster
parents [146]. Alternatively, it is accomplished via a direct production of cluster
ions through supersonic coexpansion of cluster and molecule constituents. For these
approaches, the selected cluster species may also undergo studies of metastability,
photoexcitation and dissociation, or collision induced dissociation (CID) processes,
etc. [147]. Besides, advances in the technology of flow-tube reactors (such as the ones
in Castleman and Schwarz groups) [148–150] have opened up a new realm of investigations in the past decades [147, 151, 152]. This will be introduced in detail following.
Functioning as mini-sized flow tube, tandem reaction cells (also known as collision
cells) have become popular and convenient along with pulsed buffer gas, such as
those used in Lievens group [153–155], Andersson group [156], Bowen group [157],
and Fielicke group [158], allowing extensive studies of cluster reacvity along with
multiple-photon-dissociation (MPD) spectroscopy and photoelectron spectroscopy
of in situ synthetic clusters.
2 Instrumentation for Cluster Science
of the cluster, P k /P ∞ . This equation helps explain why a cluster distribution is usually
seen with a normal or an inverse gamma distribution while opposed to a logarithmic
decay (i.e. the monomer is more intense than the dimer which is also more intense
than the trimer, etc.). This equation also indicates interpretion why metals with higher
boiling points tend to form smaller clusters even though under the same experimental
conditions (due to the reduced partial pressure of the monomer, and then Φ k ).
Also Eq. 2.6 stands to reason its time independence: once the cluster has exited
from the source it will no longer grow. The time allowed for cluster formation is
also called “aggregation time” [119] which is dependent on the parameters of the
source and the speed/pressure of the backing gas [120]. In experimental design, the
“aggregation time” can be qualitatively ascertained by the flow rate of the backing
gas and the space immediately between the target material and the exit of the source,
called “waiting room”. A practical means of increasing the “aggregation time” (i.e.,
gas pressures in the “waiting room”) is by simply enlarging the flow of the gas.
Nevertheless, the operation to simply increase the flow of the buffer gas could be
insufficient to adjust the “aggregation time”, as the gas is expanding into vacuum
and therefore small changes in flow will do rare changes to the pressure in the source
[121]. In comparison, adjusting the exit nozzle is a more efficient solution. The
conductance “C” of a tube is inversely proportional to its length “L” and directly
proportional to the fourth power of its diameter “D” according to fluid mechanics (C
∝ L
−1 ; C ∝ D
4 ) [121].
2.3 Cluster Reaction Apparatus
The investigations into cluster reactivity have been accomplished by several
approaches [122–145], for example, through the study of the ensuing dynamics of
product evolution of excited species formed on the excitation of the neutral cluster
parents [146]. Alternatively, it is accomplished via a direct production of cluster
ions through supersonic coexpansion of cluster and molecule constituents. For these
approaches, the selected cluster species may also undergo studies of metastability,
photoexcitation and dissociation, or collision induced dissociation (CID) processes,
etc. [147]. Besides, advances in the technology of flow-tube reactors (such as the ones
in Castleman and Schwarz groups) [148–150] have opened up a new realm of investigations in the past decades [147, 151, 152]. This will be introduced in detail following.
Functioning as mini-sized flow tube, tandem reaction cells (also known as collision
cells) have become popular and convenient along with pulsed buffer gas, such as
those used in Lievens group [153–155], Andersson group [156], Bowen group [157],
and Fielicke group [158], allowing extensive studies of cluster reacvity along with
multiple-photon-dissociation (MPD) spectroscopy and photoelectron spectroscopy
of in situ synthetic clusters.