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%TCIDATA{Created=Wed Sep 02 13:35:50 1998}
%TCIDATA{LastRevised=Mon May 10 10:06:44 1999}
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\begin{document}
\author{I. Labazan, D. Azinovi\'{c}, S. Gogi\'{c}, and S. Milo\v {s}evi\'{c}}
\address{Institute of Physics, P.O.Box 304, HR-10000 Zagreb, Croatia}
\title{TEMPORAL EVOLUTION OF INTERMETALLIC DIFFUSE BAND SPECTRA }
\maketitle

\begin{abstract}
\vspace{1.5in}

LiZn and LiCd vapour mixtures prepared in the heat-pipe oven are irradiated
with pulsed excimer laser emission at 308 nm. Temporal evolution of the
fluorescence spectra is observed showing that different processes occur such
as multiphoton excitation, collisional energy transfer, photochemical
reaction, relaxation etc. Temporal change of the LiZn and LiCd blue-green
diffuse bands spectral shape is observed and interpreted in terms of
previous spectral simulations. The effective lifetimes of different spectral
features are determined.

PACS 33.10. -n; 33.20.Kf; 33.80.Gj

\newpage
\end{abstract}

\section{Introduction}

Spectra from intermetallic vapor mixtures are usually quite complicated due
to the overlap of different spectral features \cite{mil95}. A general remedy
of this problem is a careful preparation of the alloy composition \cite
{azi93}, \cite{azi95}, \cite{azi96}. LiZn and LiCd diffuse band spectra were
first reported in Ref. \cite{mil92}, \cite{hem92}. These experiments were
performed with the pulsed laser but only time-averaged spectra were
recorded. Subsequent detailed spectral simulations have shown complex nature
of the diffuse band spectra \cite{li94}. Both bound-bound and bound-free
types of transitions were found to play an important role.

When pulsed lasers are used for excitation, different spectral features can
be distinguished by performing temporal analysis of spectra with nanosecond
resolution \cite{azi96a}. The temporal analysis of alkali-IIB excimer bands
was used in the case of NaCd and NaHg to identify molecular formation
channel and relevant rate coefficients \cite{gru94},\cite{gru96}.

In this work we have studied temporal evolution of intermetallic diffuse
bands, particularly of the LiZn and LiCd systems produced photochemically in
the reaction:

\begin{equation}
Li_{2}(2^{1}\Pi _{u})+{\rm IIB}\longrightarrow Li{\rm IIB}(2^{2}\Pi )+Li.
\label{Eq.1}
\end{equation}
where {\rm IIB} is Zn or Cd atom. Under conditions of dense vapour mixtures
reaction (\ref{Eq.1}) is followed by the thermalization of the nascent
population distribution of the 2$^{2}\Pi $ state

\begin{equation}
Li{\rm IIB}(2^{2}\Pi ,v^{\prime }J^{\prime })+{\rm IIB}\longrightarrow Li%
{\rm IIB}(2^{2}\Pi ,v^{\prime *}J^{\prime *})+{\rm IIB}  \label{Eq. 2}
\end{equation}
which is responsible for the change in the diffuse bands spectral shapes.

We use excimer laser at 308 nm to excite Li$_{2}$(C$^{1}\Pi _{u}$) and
temporal analysis of different spectral features to clarify excitation
processes and photochemical production of LiZn and LiCd molecules.
Illustration of the processes (1) and (2) is shown in Fig. 1 for the LiCd
case.

\section{Experiment}

The experimental arrangement was similar to that described in our previous
paper \cite{azi96a}. Metal vapor mixtures were generated in a heat-pipe
ovens. The heat-pipe ovens were filled with approximately same amounts of
lithium and zinc or cadmium. The temperature in the central region of the
heat-pipe oven was varied from 1020 K to 1220 K. Helium or argon were used
as buffer gases at pressures from 5 to 100 Torr. The heat-pipe mode of
operation is established for zinc or cadmium vapour. The preparation of
electronically excited Li$_{2}$ molecules in specific rovibrational levels
of the 2(C)$^{1}\Pi _{u}$ state is achieved by means of the 20 ns pulsed
XeCl excimer laser lines at 308 nm (LPX 105E). Laser beam (initially of
shape 2x1 cm) was spatially filtered and made parallel with a quartz
telescope, having the diameter of 2-3 mm in the center of the heat-pipe
oven. The horizontal laser induced fluorescence image was rotated by a Dove
prism and focused to the vertical entrance slit of the monochromator. The
resolution of the system was of about 0.1 nm. The signal from the
photomultiplier with S20 cathode was averaged by means of a boxcar averager
(Stanford SR245). The analog output from the boxcar was converted to digital
signal through an A/D converter and fed to a laboratory computer. Temporal
analysis is performed by means of a box-car averager or a 8-bit, 150 MHz
digital storage oscilloscope (Hameg 1507). Spectral response of our
detection system was measured by using a standard calibrated tungsten lamp
and is found flat in the region of interest (450-550 nm).

We performed two types of measurements. To measure laser induced
fluorescence we scan the monochromator in wavelength interval of interest,
setting a specific gate and time delay value of the box-car averager. Time
resolved intensity of the fluorescence is measured at a given setting of the
monochromator by using a digital storage oscilloscope or by scanning the
time delay of a box-car gate. Zero of the time scale is set to the rising
edge of the laser pulse.\newpage

\section{Results and discussion}

\subsection{Excitation of Li-Zn vapour mixture}

Figure 2 shows the fluorescence of the LiZn vapour mixture in the region
from 420 to 500 nm, excited by the laser lines at 308 nm. For short time
delay of 10 ns (Fig. 2a) we observed only the LiZn excimer band (2$^{2}\Pi
-1^{2}\Sigma $ transition$)$, peaked at 477 nm \cite{mil92}. At the delay of
40 ns we noticed Zn atomic lines at 472.2 and 481.2 nm (Fig. 2b) (5$^{3}$S$%
_{1}$ - 4$^{3}$P$_{1}$ and 5$^{3}$S$_{1}$-4$^{3}$P$_{2}$ transitions). For
the delay of 70 ns (Fig. 2c) Zn lines decreased substantially. This time
behavior suggests that Zn atoms are populated in near resonant two-photon
absorption process $Zn(4^{1}S_{0})+2h\nu +M\longrightarrow Zn(6^{3}S_{1})+M,$
where M is Zn or Li atom, followed by a quick cascade emission $%
Zn(6^{3}S_{1})\longrightarrow Zn(5^{3}P_{1})\longrightarrow Zn(5^{3}S_{1})$.
Proposed excitation mechanism of zinc atomic emission is illustrated in Fig.
3. Alternative excitation mechanism which involves energy-pooling collision
4p $^{3}$P$_{J}$ + 4p $^{3}$P$_{J}$ can be excluded on the basis of temporal
evolution \cite{ume90}. Energy pooling excitation results with microsecond
decay times. As zinc atomic features decrease, Li atomic lines at 460.3 and
497.2 nm appear (4D-2P and 4S-2P transitions) in spectra. Lithium atomic
spectral features are consequence of the dissociative recombination of Li$%
_{2}^{+}$ , the latter being produced via $Li_{2}(C^{1}\Pi _{u})+h\nu
\longrightarrow Li_{2}^{+}+e$ process.

Figure 2d shows spectrum with large gate of the box-car averager which
should be compared with the time averaged spectra (see Ref. \cite{mil92}).
Following the sequence from a to c, we observe change of the LiZn band
shape. The band tail extending from 470 nm to 440 nm, visible also in the
time-averaged spectrum (Fig. 2d), is most pronounced for earliest time
delays. The ratio of intensities at band maximum (477 nm) and band tail (455
nm) changes from 1/4, 1/7 to 1/14. According to spectral simulations
presented in Ref. \cite{li94} this long tail is of bound-free nature and
originate from higher vibrational states of the 2$^{2}\Pi $ state (v'$\geq $%
7). Therefore observed change in the band shape indicate relaxation of the
nascent population distribution which is obviously centered at high
rovibrational levels of the 2$^{2}\Pi $ state.

The temporal evolutions at a) 477.5 nm (LiZn), b) 339.4 nm (Li$_{2}$ C
state) and c) 460.3 nm (Li) are shown in Fig. 4. We have determined the
effective lifetimes of the corresponding emissions, which are at T= 1050 K 12%
$\pm $5 ns for LiZn(2$^{2}\Pi $), 10$\pm 3$ ns for Li$_{2}$(2$^{1}\Pi _{u}$)
and 19$\pm 3$ ns for Li(4$^{2}$D) state. With increasing temperature the
metal vapor pressure increases causing the LiZn(2$^{2}\Pi $) effective
lifetime to decrease linearly, i.e. at 990 K $\tau _{eff}$=20$\pm 5$ ns and
at 1060 K $\tau _{eff}$=8.5$\pm 4$ ns. The lifetime is approximated by the
extrapolation to the zeroth zinc vapor pressure at 723 K, which gives $\tau
_{rad}$=(65$\pm $26) ns.

\subsection{Excitation of Li-Cd vapour mixture}

Figure 5 shows spectra of the Li-Cd vapor mixture excited by 308 nm laser
lines at T= 1030 K and a) $\Delta $=25 ns and G=10 ns, b) $\Delta $=50 ns
and G=10 ns, c) $\Delta $=70 ns and G=75 ns and d) time-averaged $\Delta $%
=10 ns and G=200 ns. Excited LiCd(2 $^{2}\Pi $) molecules are produced
through the photochemical reaction \cite{li94} with nascent rovibrational
population distribution far from Boltzmann distribution (see Fig. 1), which
gives dominant bound-free emission for short time delays after the laser
pulse (Fig. 5a). Subsequent collisions with Cd atoms are efficiently
thermalizing the nascent rovibrational population. This is observed, for
larger time delays (see Fig. 5b and 5c), as the change of spectral shape of
the blue-green diffuse band, with dominant bound-bound contribution to the
emission \cite{li94} (sharp peaks near the band maxima). In the present
experiment we do not observe cadmium atomic lines in the spectral region
shown in Fig. 5, neither the characteristic emission at 326.1 nm (5p $^{3}$P$%
_{1}$- 5s $^{1}$S$_{0}$ transition). The latter was observed in Ref. \cite
{ume90a} for Cd-He mixtures where 308 nm excitation was explained to proceed
through the characteristic features of the Cd-He collision complex. Although
one could expect rather similar structure of the Cd-He and Cd-Ar or Cd-Li
complexes they are quite different in respect to the efficiency of specific
collisional energy transfer process. This shows that careful preparation of
metal mixtures and choice of a buffer gas can be used to efficiently promote
different spectral features. Lithium atomic lines appear after 70 ns
similarly as in the LiZn case. We note that, as in LiZn case, Li$_{2}$
spectral features (diffuse band at 458 nm) are absent from spectra. This is
an indication of the low Li and Li$_{2}$ density in the vapour mixture above
liquid alloy.

Figure 6 shows LiCd diffuse band spectra in the region 450 - 500 nm recorded
at the time delay of 75 ns for different buffer gas (Ar) pressures. We
assume that for all cases the heat-pipe works in the heat-pipe mode for
cadmium, namely central heated part mainly contains cadmium vapours with
small portion of lithium vapour (few percent), while buffer gas fill space
between heated zone and windows of the heat pipe. Therefore the change of
the diffuse band shape is caused by collisions given by process of type (2).
For a low buffer gas pressures the diffuse band shape preserve bound-free
nature even at large time delays.

Figure 7 shows the time evolution of different spectral features obtained by
means of a digital oscilloscope. Set of such curves have been measured for
different buffer gas pressures. From each curve decay times were determined
by performing single-exponential fit starting at about 60 ns (falling edge
of the laser pulse). This is done in order to avoid influence of the laser
pulse width. Plotting effective radiation rates (inverse of decay time)
versus Cd atom density allows us to determine the total quenching rate
coefficients for relevant upper levels (see Figure 8). Data for zero cadmium
density are obtained from measurement in the pure lithium heat-pipe oven at
the same temperature and buffer gas pressure. We note that emission from 2P
level is strongly influenced by radiation trapping. We obtain quenching rate
coefficient for the Li(2P) level to be (3$\pm 1)\cdot 10^{-12}$ cm$^{3}$s$%
^{-1}$ and for the C $^{1}\Pi _{u}$ (v'$\approx $ 13-19) Li$_{2}$ levels (1.2%
$\pm 0.5)\cdot $10$^{-11}$cm$^{3}$s$^{-1}.$ The effective lifetimes of LiCd
spectral features at 489 nm and 482 nm were determined to be 15 ns and 14 ns
respectively. We did not observe significant change of the effective
lifetimes with atom density as in the LiZn case.

\newpage

\section{Conclusions}

We have shown that, in spite of the relatively large collision frequencies
for total pressures usually used within the heat-pipe ovens, nanosecond time
spectral analyses can provide important information for understanding of
complicated processes in intermetallic vapor mixtures. Photochemical
production of the LiZn(2$^{2}\Pi )$ and LiCd(2$^{2}\Pi )$ excimers by means
of 308 nm laser photons results with population of high vibrational states.
By performing temporal spectral analysis we observed the redistribution of
the population towards the lower vibrational states which completes after
the time delays larger then 50 ns. The observed changes in the diffuse band
shapes are consistent with predictions of detailed spectral simulations
given in Ref. \cite{li94}. The thermalization effect is especially
pronounced in the case of LiCd through appearance of bound-bound structures,
which originate from lower vibrational levels in the upper state. Measured
effective lifetimes of the LiZn and LiCd emission are similar as those
reported for NaCd and NaHg excimers \cite{gru94,gru96}.

\vspace{1.5in}

Acknowledgment

This work was financially supported by the Ministry of Science and
Technology of the Republic of Croatia. We wish to thank Dr. Goran Pichler
for valuable discussions and continuous support of this work.

\newpage

FIGURE CAPTIONS

Fig. 1: Partial potential energy diagram of Li$_{2}$ and LiCd molecules
showing 308 nm excitation, photochemical reaction path, relaxation process
and LiCd bound-free and bound-bound type of emission.

Fig.2: The spectra of the Li-Zn vapor mixture excited by 308 nm laser lines
at Ar pressure of 52 Torr and temperature T= 1050 K. Time delay and gates
are: a) $\Delta $=10 ns, G=10 ns, b) $\Delta $=40 ns, G=10 ns, c) $\Delta $%
=70 ns, G=100 ns, and d) $\Delta $=1 ns, G=300 ns.

Fig. 3: Term diagram of zinc atom illustrating excitation mechanism of a
high lying levels by 308 nm photons. Data are taken from Ref. \cite{moo52}.
The energy defect for the first step is about 30 cm$^{-1}$ and for the
second step 490 cm$^{-1}$.

Fig. 4: Temporal evolution at a) LiZn band (477.5 nm), b) Li$_{2}$ C$^{1}\Pi
_{u}$-X$^{1}\Sigma _{g}^{+}$ band (339.4 nm), and c) Li 4$^{2}$D-2$^{2}$P
atomic line (460.3 nm). Temperature T = 1050 K, and argon buffer gas
pressure 11 Torr. The full curve represents a double-exponential fit of the
experimental values.

Fig. 5: The spectra of the Li-Cd vapor mixture excited by 308 nm laser lines
at Ar pressure of 100 Torr and temperature T= 1030 $K.$ Time delay and gate
are: a) $\Delta $=25 ns, G=10 ns, b) $\Delta $=50 ns, G=10 ns, c) $\Delta $%
=70 ns, G=75 ns, and d) $\Delta $=10 ns, G=200 ns.

Fig. 6: The spectra of the Li-Cd vapour mixture excited by 308 nm laser
taken with time delay $\Delta =$75 ns and gate G = 60 ns for different
buffer (Ar) gas pressures. T = 1000 K.

Fig. 7: Temporal evolution of different spectral features from LiCd vapour
mixture irradiated by 308 nm laser. From the top to bottom fluorescence is
shown for: two LiCd diffuse band maxima, 2P-2S lithium transition, Stokes
emission within C-X manifold and scattered light of the laser pulse.
Temperature was 1040 K and buffer gas pressure 5.6 torr.

Fig. 8: The effective radiative rates of a) Li(2P) and b) Li$_{2}$(C-X) band
versus cadmium atom density for T = 1040 K. The natural radiative rate of
Li(2P) is 3.69$\cdot 10^{7}$ s$^{-1}$ \cite{mca96}. The natural radiative
rate of the C-state is about 1.6$\cdot 10^{7}$s$^{-1}$ \cite{wey96}.

\newpage

\section{References}

\begin{references}
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\end{references}

\end{document}
