17
Internarional Journal of Mass Spectrometry and Ion Processes 126 ( 1993) 1l-24
0168-l 176/93/$06.00 0 1993 - Elsevier Science Publishers B.V., Amsterdam
Mechanisms for the desorption of large
organic molecules.
Part 2*
R.E. Johnson
Engineering
Physics,
Thornton
(Received 10 February,
Hall, University
of Virginia, Charlottesville,
VA 22903, USA
1992; accepted 15 November 1992)
Abstract
Models are reviewed for ejection of intact biomolecules
and biomolecular
ions in response to the electronic energy
deposited by a fast heavy ion. These models are compared with laboratory
data and molecular dynamics calculations.
Over a limited range of electronic energy deposited per unit path length, the scaling of the total yield with energy
deposition is understood,
and it has been shown that, except for Langmuir-Blodgett
films of fatty acids, the large intact
ions are ejected primarily from the surface or adsorbed sites. The principal outstanding
problems are a detailed description
of the electronic energy conversion
into molecular motion and the ion formation-neutralization
process. An understanding of these is needed in order to improve sensitivity at the higher masses in PDMS.
Key wordx Plasma
desorption
mass spectrometry;
Langmuir-Blodgett
Introduction
In Part 1 [l] the data and models for ejection of
intact biomolecules from surfaces due to the energy
deposited by an impacting fast heavy ion were
reviewed. At that time the models available
appeared to be inappropriate for describing the
new data from the Uppsala group on ejection of
neutral molecules from a sample of leucine [2] and
the data obtained by a number of groups [3-51 for
ion ejection from Langmuir-Blodgett (LB) films of
fatty acids. Both types of data indicated that a
volume of material was ejected, whereas most
models at that time considered, in one way or
another, the sublimation of surface layers [l].
Although this in itself was not necessarily contra’ Paper presented at the 6th Texas
Spectrometry,
Gasp&, Que., Canada,
Symposium
IS-19 June,
on Mass
1992.
films; Biomolecules.
dictory, the small amount of data on the scaling of
the yields with energy deposition also suggested the
models were not correct.
Subsequent to publication of Part 1 [l], W. Ens
et al. [6], working with the Uppsala group, showed
that the angular distribution of ejected intact large
molecular ions exhibited a unique signature. This
was a major breakthrough because it indicated the
ejection process was impulsive, and this result has
been used to discriminate between models for
intact ion ejection. Later it was also shown that
the angular distribution of the intact ions ejected
from LB films [7] differed from that for films of
other biomolecules, probably because of the elongated shape of the molecules and their order in LB
films. The difference in the ejection angle distribution at first appeared to be consistent with the
difference observed earlier between LB films and
leucine samples [2-31: a difference in the scaling
18
of the apparent volumes ejected with electronic
energy deposited. However, recent data by the
Erlangen group [5] for the LB films suggests that
the differences in scaling of the yields with the
electronic stopping power, (dE/dx), were due in
part to the fact that the ions were not incident
normal to the surface in the experiments on LB
films. Whereas non-normal incidence does not
drastically change the scaling of the yield with
energy deposition in PDMS for multi-layers of
most molecular species [8,9] the elongated shape
of the ordered fatty acid films aparently affects
the material response at angular incidence.
There now appears to be some agreement that
the total volume of material ejected scales as
(dE/dx)E with n x 3 over a limited range of
(dE/dx),. This result should still be accepted with
caution because the leucine data is limited and for
LB films only ion ejection has been measured.
However, it is presumed by most writers that the
ejected intact ions from LB films of fatty acids
roughly represent the total ejecta from an LB film
of these peculiar molecules.
A damaged volume due to fast heavy ion impact
has also been seen experimentally for the track
forming material [lo] mica using an atomic force
microscope to observe the “crater” (hollow)
formed. In additon, since Part 1 [l] a number of
molecular dynamics (MD) simulations have been
performed [7,1 l-l 51 to describe at the molecular
level the process of ejection of material caused by
exciting a cylindrical track of molecules. These
were initiated by Hilf and Kammer at Oldenburg
[14] and Fenyii et al. [7,1 l] at Uppsala; subsequently, such calculations were carried out by
Urbassek et al. [15] and our group at Virginia
[ 12,131. These calculations confirmed that the ejection of a volume of material occurs and that the
scaling of the volume ejected with (dE/dx), is very
similar to that found experimentally.
Bitensky and Parilis [16] first suggested that both
volume ejection and the ionization process could be
understood in terms of the formation of a shock
wave by the energy deposited by an incident ion
and the intersection of this shock wave with a
R.E. Johnson/M.
J. Mass Spectrom. Ion Processes 126 (1993) 17-24
surface. Whereas their analytic model gave reasonable agreement with the biomolecule ion yields, it
did not describe the total yield. Johnson et al [17]
modified their model, extending earlier ideas about
the ejection of molecules from low temperature
condensed gas solids. The modified model was
referred to as the “pressure pulse” model because
the net impulse produced by the transiently pressurized cylindrical track was used to calculate the
volume ejection. This simple concept led to an
approximate analytic model which has subsequently been tested against experiment and, in
much greater detail, against the MD results [18].
Recently its relationship to the theory of weak
shocks [ 191was discussed. Subsequently, Bitensky
et al. [20] modified their model, also using momentum transport to inititate the ejection of neutrals.
In this paper the diverse and as yet sparse set of
experimental data and the more extensive set of
MD calculations are briefly summarized and compared with the analytic expressions for the yield.
Following Part 1 [l], two principal points of
success and failure are pointed out. The latter
primarily relate to the very important unsolved
problems: the details of the conversion of electronic energy into energy of motion, which should not
depend strongly on material properties, and the
nature of the ionization process, which is very
material-dependent. Readers are also referred to
a number of excellent reviews [8,9,21,22] for
many of the details.
MD calculation
Molecular dynamics calculations of the evolution of a solid (t < lo-” s) in which a cylindrical
track of material is “excited” (energized in some
manner) have been performed on the following
samples: atoms (40~) interacting by van der
Waals pair potentials and excited by giving each
atom in the track a kinetic energy in a random
direction [15]; massive (M lOOO- 10 000 u) particles with hard cores bound together in the solid
by van der Waals pair potentials and excited
by expansion of the size of the core [7,11,18];
R.E. JohnsonlInt.
J. Mass Spectrom.
Ion Processes
126 (1993)
19
17-24
Fig. 1 a steep dependence is seen on (dE/dx),n.
For a material with cohesive (sublimation) energy U
1.13 4,
0.56 /
4
2.82
1
I
10
100
1
1000
( dE/dx Jeff ( a/U )
l
Fig. 1. Yield in atoms (molecules) removed per “ion” incident
plotted vs. (dE/dx),n
scaled to the average atomic (molecular)
size I and the cohesive energy per molecule U. The effect of the
incident ion in each case is simulated by exciting a cylindrical
region uniformly. In Fenyii et al. (7,l l] large “molecules”
are
expanded; in Banerjee et al. [12] diatomic molecules are vibrationally excited; in Urbassek et. al. 1151 (all other calculations)
Ar atoms are given random kinetic energy; these curves are
labeled by the radius of the cylindrical region Ro, scaled by the
average lattice spacing I = n,-I” Note that in the latter results
the excitations
are different from those in Fenyii et al. and
Banerjee et al., in which the molecules have physical size giving
a less compressible material at high pressures. At low (dE/dx),n
a cubic dependence
is seen and for R,,/I = 2.82 or greater in
Urbassek et al. [15], for higher (dE/dx),s
and/or small Roll
they find a linear dependence on (d.E/dx),s. (Taken from ref. 15.)
diatomic molecules [32 u] excited vibrationally having internal Morse potentials and with van der
Waals pair potentials acting between atoms on
neighboring molecules [ 12,131; linear molecules in
two dimensions excited by expansion and intended
to represent an excited LB film [14]; and a one
dimensional chain of molecules [23].
The three dimensional calculations above all
lead to “crater formation”. A summary of the
yields for these models vs. (dE/dx),rf are given in
Fig. 1. By (dE/d x )eff is meant the effective expansion energy per unit depth in the excited cylindrical
region. Presumably this scales with (dE/dx),:
where f is the fraction
(dE/dx),n =f(dE/dx),
of the initial electronic energy deposited that
produces expansion. For molecular materials in
Where p and z are the average radial extent and
depth of the volume ejected, n, is the molecular
number density, and 1 = n,113 is the average molecular size. The dependence on (dE/dx),n to the
third power can be roughly understood if each
dimension of the ejected volume scales as
(dE/dx),n. In the calculation for an atomic solid,
it is seen that a steep onset occurs dependence at
low (dE/dx),fr for each cylindrical radius assumed,
which gives way to a much slower increase with
(dE/dx),n
at higher excitation densities (i.e.
Y 0: (dE/dx),n). Using the first equation in Eq.
(I), the quantity 2 appeared to change slowly at
high (dE/dx),n, but the quantity G was still found
to be approximately proportional to (dE/dx),.
Those calculations were terminated, necessarily,
at tx lo-” s, a time at which the crater had not
relaxed and the walls of the craters were still highly
compressed. In fact, the result at high (dE/dx),n
may be consistent with standard “shock” models
for ejecta production and crater formation in other
impact phenomena. It differs from the other calculations in that the small atomic species can be compressed much further than the molecular species
which have significant physical size [ 12,13,15].
Experimental
data
The data on total yields is extremely limited. The
only direct measure of the total yields for biomolecules is that of the Uppsala group [2]. In that
experiment the ejected intact leucine molecules
were collected and counted by amino acid analysis
and assumptions were made about the ejecta angular distribution. The total intact yield varies
roughly as (dE/dx):, although this dependence
must be accepted with caution because only four
data points were obtained [8]. The measured total
yields are compared to the ion ((M + H)+ and
(M - H)-) yields in Fig. 2, clearly showing that
20
R.E. Jbhnsonjlnt. J. Mass Spectrom. Ion Processes 126 (1993) 17-24
l__A_.--
--
IO'
( dE/dx )e
( MeVimgicm*
102
)
I
0
Fig. 2. The yields vs. (dE/dx),
for intact leucine molecules
molecular ions. (Taken from ref. 2).
and
ion and total intact ejecta yields can scale differently with (dE/dx),. This point is often made but
equally often ignored in developing models. The
slower dependence of the ion yields for such
samples is due, in part, to ion ejection being predominantly a surface process, as discussed below.
In contrast to this, a remarkable result is that bombardment of a LB film of fatty acids results in ions
being ejected from depth, as determined by marker
layers [3-S]. The Erlangen group [5] recently
showed that for fast heavy ions at normal incidence the average depth of origin x is roughly
proportional
to (dE/dx),
consistent with the
above discussion of the scaling of the total yields.
The radial scaling is less certain, although conical
shapes have been proposed [3,5,21]. The nature of
the depth scaling was obscured earlier because nonnormal incidence was used in most LB experiments, with z
varying as a low power of
(dE/dx), (n < l/2 at 45°C) [3].
The “crater” (hollow) produced in a mica film by
fast heavy ion bombardment has also been measured and is of interest, although mica is a rather
different material [lo], and the crater defines a
softened region. The actual measured dimensions
were found to vary with tip force, but the dimensions of the craters observed scaled (roughly) with
(dE/dx), at the higher energy depositions studied.
This is shown in Fig. 3 for the crater diameter
measured with a given force on the tip. What is
seen is a region at the highest values of (dE/dx),
in which the crater diameter is nearly linear in
10
20
(dE/dx
)e
a
/
L I
(
;
30
40
50
60
[ MeV
.mg-’ .cm”]
Fig. 3. Diameters of hollows (craters) produced in mica by heavy
fast ions vs. (dE/dx),. The size is affectd by the force on the tip
of the atomic force microscopic.
At largest (dE/dx),
the diameter varies roughly linearly with (dE/dx),. A threshold is seen
at low (dE/dx),.
(dE/dx), (the scaling observed for the biomolecule yields). However, at low (dE/dx), there is a
steeper “threshold” region for this refractory solid.
A steep “threshold” behavior is also seen for the
total ejection yield from low temperature solid Ss
[24] with light fast ions.
The existence of a “threshold” region for intact
molecular ejection was clearly established by
Hakannson et al. [25] for ion ejection. The Erlangen group [26] has found good fits to measured ion
yields using an excitation density “threshold”
(dE/dx)thres. That is, they write Y c( [(dE/dx),(dE/dx),hr,,]“. It is important to remember that
both the ejection process for large intact molecules
and the energy deposition process are necessarily
statistical, as discussed in Hedin et al. [27] and in
Part 1 [l], so there is no cut-off in (dE/dx), for the
total yield. Further, the yields in Ref. 26 are ion
yields and not total yields.
Analytic models
For the cylindrical excitation geometry produced
by a fast heavy ion, the various models can be
grouped into three classes of dependencies on
(dE/dx),n where (dE/dx),* is some fraction of
(dE/dx), acting to cause expansion, as discussed
above. First, “thermal spike” models, for which
(dE/dx),r is converted to a “temperature”, give
R.E. JohnsonlInt. J. Mass Spectrom. Ion Processes 126 (1993)
21
17-24
the following scaling in Eq. (1): (~2) cx [(dE /
dx)/n,U], and z 0: ,[l(dE/dx),&],
except near
where the yield decreases more
“threshold”,
rapidly with (dE/dx),n [l]. Allowing an evolving
surface (i.e. crater formation) may alter this dependence. Such models were discussed extensively in
Part 1 [l] and all had the property that the “depth”
increased faster than the radial size of the ejecta
region. The second type of dependence is given
by the “shock wave” models of Yamamura [28],
Carter [29], and Bitensky and Parilis [16], in
which (dE/dx),M represents the kinetic energy left
behind by the passing shock. In such models the
radial scale of the ejected volume is found by
assuming that this energy can be spread over an
area determined by the sublimation energy U i.e.
wp2 K [(dE/dx),R/n,U],
identical to the spike
model above. In addition, it is assumed in these
models that z also scales as the radial extent of
the region [(dE/dx),n/n, U]‘j2, giving a hemispherical crater. Finally, the “pressure pulse” model
[17,30,31], in which (dE/dx),ff produces a radial
and out-of-the surface expansion, and the revised
“shock” model of Bitensky et al. [20] both consider
the transport of momentum to the ejected volume.
In these models each dimension of the ejected
volume roughly scales as I[Z(dE/dx),n/U].
The “pressure pulse” model has been shown to
describe extremely well the results of the MD simulations [18]. For a van der Waals solid of large
structureless particles, the yield is written
(2)
where 1= n-,‘13 (the molecular size) and c is a
model-dependent constant. In Johnson et al. [17]
c x 4 x 10m5 for structureless point particles,
which is roughly consistent with the MD results
[11,18]. (Note that it was stated in Ref. 11 that
the agreement in the size of the yield between the
analytic and MD calculations was quite close when
the constant p in the analytic model (related to
specific heat) was that for a gas of structureless
particles; in fact a value of about twice that is
needed.)
Based on the MD calculations, the lattice expansion energy required to produce the leucine yields
(dE/dx),R =f(dE/dx),
gives f= 0.01 [ll]. (Note
that the MD results were for more massive particles, but the yields appeared to be independent of
mass over the limited range tested and so were
applied to leucine.) In Part 1 [l] it was pointed
out that in such materials excited by fast heavy
ions most of the electronic energy is indeed available to be converted into expansion, either by
repulsive relaxation [32] or by internal excitations
[33]. Therefore, because (dE/dx),n is a lattice
energy the increase off divided by the lattice specific heat gives an estimate of the effective internal
“heat capactiy” [22] at the time of ejection, including electronic and vibrational modes. That this is
much larger than the specific heat of leucine means
that a large fraction of the energy is still in electronic excitation at the time of ejection.
Before proceeding it is important to note that the
models above all exhibit “threshold” regions. The
“threshold” dependence for “thermal spike” models is described in Part 1 [l]. For the “pressure
pulse” models an aspect of this region has been
discussed recently [34]. It is clear that the radial
size of the ejected volume (the volume receiving
the momentum) must exceed the larger of the molecular size 1 or the radial size of the highly fragmented core [27]. Applying this, the radial size of the
ejected volume now scales as ,{[/(dE/dx)&
U] - l}, because at low (dE/dx),R the molecular
size is the important quantity [25,27]. This gives an
effective “threshold” energy density determined by
the cohesive energy U as
(dE/dx),h,,
a (u/l)f
(3)
a form used earlier [l]. Although the threshold
values of Brand1 et al. [26] only apply to ion ejection, the values for valine are equivalent to
f= 0.03, roughly consistent with the value above
derived by Fenyii et al [l 11.
General consideration: ion formation
A point of disagreement
between the analytic
22
model [17], in which the molecules do not have size,
and the MD simulations of FenyG et al. [ 11,181is in
the angle of ejection of intact ions [32]. The MD
results first gave the correct sense of the angular
distribution [7,11] but not the correct average
angle. Quite remarkably, for intact large biomolecules Johnson et al. [17] showed that the analytic
model predicted quite accurately the average angle
of ejection for intact ions measured by Ens et al. [6].
This was only the case if the biomolecule was an
absorbed species [35]. Bitensky et al. [20] subsequently showed the full angular distribution compared well with the full angular distribution
measured by Wein and co-workers [36] for dimers
of valine. Again, this agreement is only obtained if
these ejected ions are assumed to be primarily
formed from adsorbed or surface molecules, a
point not made clear in their paper. Based on this
it is clear why the MD calculations of the “ion”
yield do not describe the angular signature of the
intact ions very accurately: the intact ions ejected
are rare events and the MD calculations describe
the dominant ejecta. In the analytic model only the
ejection angles of the surface (absorbed) molecules
can be predicted because these species interact
weakly with neighbors, whereas even species
leaving from the first layer are deflected from
the direction given by the initial impulse [7] because
of the physical volume they occupy in the surface
layer.
That a significant fraction of the intact ions of
large biomolecules are derived from adsorbed sites
was suggested experimentally by the fact that
adsorption of biomolecules on nitrocellulose gave
ejection angle dependences nearly identical to those
for ejection from multilayers for large biomolecules
[7,8]. Conversely, small biomolecular ions (monomers of valine and fragments) do not exhibit
the same ejection angle signature [31,35,36]
and, therefore, in addition to absorbed sites, these
ions may be thermally ejected or be derived from
molecules within the surface layer and, possibly,
sublayers.
As pointed out in Part 1 [l], the total yield for a
given distribution of energy deposition can be
R.E. JohnsonlInt. J. Mass Spectrom. Ion Processes 126 (1993) 17-24
written as
YZZ
ss
@,(p’,4 d2pdt
(4)
where (p, is the flux of sputtered material as a function of time t and of the radial distance p from the
incident ion track. In order to understand the scaling with (dE/dx), this expression was approximated [l] above the “threshold” region as in Eq.
(1). It is now tempting to write the ion yield as
Yi M Pi Y, as is often done. But it is seen in Fig. 2
that this is not necessarily correct. Therefore, it was
suggested by Hedin et al. [27] and in Part 1 [l] that
Yi M Pi(.rrp2)iaZi
(5)
where Pi is the probability of formation of the ion
and its survival to detection, and the quantities Zzi
and 7rz for the ion yield can differ from the quantities in Eq. (1) for the total yield.
Although Bitensky et al. [16,20] claim to use an
expression like Yi z Pi Y to match both the scaling
of the ion and neutral yields with (dE/dx),ff they in
fact use different “shock” criteria [20], (as discussed
above) for ion and neutral ejection. This essentially
is equivalent to saying that (~~)i and (Az)i in Eq. 5
differ from ~7 and % in Eq. (1). For LB films of
fatty acids ionic species clearly come from depth
and, therefore, it may be acceptable to use
Yi M pi Y. There is however, no direct experimental confirmation of this assumption.
Hedin et al. [27] use an expression like that in Eq.
(5) and calculate Pi vs. (dE/dx),. They show that
the molecular size determines the “threshold”
behavior. Although their statements on the ejection mechanisms have not been confirmed, it is
clear from the data in Fig. 2 that a large volume
is ejected and the ions come from rapidly ejected
surface species within the ionization region calculated in that paper. Therefore, the description of pi
in Hedin et al. [27] is still valid. Other descriptions
of Pi [14,20] also need further consideration.
Neutralization by electron capture from the
surface is an efficient process for quenching ions,
so it is reasonable to assume that the intact biomolecular ions come predominantly from the sur-
R.E. Johnson/M
23
J. Mass Spectrom. Ion Processes 126 (1993) 17-24
face. When this is the case, it is appropriate to use
Azi M I ~ nm113[1,17,27], the size of an average
monolayer in Eq. (5). Therefore, any dependence
of Yi on (dE/dx)e comes about via Pi or (rF)i.
Hedin et al. [27] have shown that, above the intact
ion “threshold” region, the area from which intact
positive ions derive is proportional to (dE/dx),.
The dependence they calculate is due to the initial
distribution of ionization produced by the incident
ion, but they point out that the thermal spike scaling behaves similarly as discussed above and as also
noted by Luchese [23]. In contrast, the dependence
seen in Fig. 2 for negative ions is not yet explained.
Negative ions involve attachment [8,22,37] (electron or H-) so these ions may also predominantly
come from the surface region for most organic
films. If this is the case, then the quadratic dependence in Fig. 2 would imply (nT)i 0: (dE/dx)z
assuming that 4 is nearly independent
of
(dE/dx)e well above threshold, i.e. Pi constant.
Such a dependence would suggest that these ions
can form and be ejected uniformly from the surface
area associated with the total ejecta. Experimental
data is needed to test such a conclusion.
Conclusions
Although data useful for determining ejection
mechanisms is surprisingly sparse (considering
that Mcfarlane and Torgerson [38] discovered
PDMS over 15 years ago) the scaling of the total
yields with (dE/dx), appears to be roughly cubic
over a limited range of (dE/dx), due to an impulsive ejection process. The lack of an appropriate
angular signature for ionic monomers of relatively
small molecules (e.g. valine) probably implies that
“thermal” ejection [22] occurs for such species.
However, with the exception of LB films of fatty
acids, the ion yields scale very differently with
(dE/dx),. This is apparently the case as the data
of Ens et al [6]. and Wein and co-workers [36]
clearly show that the large intact ions are created
primarily from adsorbed or surface species. It has
also been shown that the calculation of the probability of positive ion formation and survival to
detection is determined, not surprisingly, by the
initial ionization density produced by the incident
ion. Although this gives the correct dependence on
and molecular size, a quantitative
(dE/dx),
description of the size of the positive ion yield is
lacking and the negative ion yield is not yet understood.
The magnitude of the impulsive energy needed
for ejection, as determined by the cohesive
(sublimation) energy is now roughly known, as
described above. Although, it was pointed out in
Part 1 [l] that most of the electronic energy deposited by a fast heavy ion is likely to be converted into
energy which leads to expansion, the details and, in
particular, the time scale of the conversion of the
initially deposited energy into expansion energy,
are not understood. Improvement in our understanding of this conversion and an understanding
of the ionization process is needed to suggest new
substrates and/or sample preparation techniques in
order to further improve PDMS.
Acknowledgments
The authors acknowledge the support of the
NSF via grant AST-91-20078, an NSF travel
grant, and travel funds from the Swedish National
Science Foundation.
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