fulltext

Fast and ultrafast fragmentation and reaction dynamics

in molecular clusters



)

C. P. SCHULZ, W. RADLOFFand I. V. HERTEL(  

)

Max-Born-Institut - Rudower Chaussee 6, D-12489 Berlin, Germany (ricevuto il 28 Ottobre 1997; approvato il 15 Ottobre 1997)

Summary. — Two prototype photochemical reactions were studied by femtosecond pump-probe technique: the fragmentation and internal protonation of ammonia clus-ters and the energy flow after photoexcitation in theNaNH

3complexes. It was found

that the ~

A state of clusters(NH3) n

decays within a few 100 fs and neutral excited

(NH3)

n,2

NH4 fragments are formed. Besides the fragmentation an ultrafast

rear-rangement within the ammonia cluster (“internal protonation”) is analysed. By compar-ison of one- and two-colour pump-probe experiments a fast dissociation of the2

E(4p)

state in theNaNH3complex intoNa(3p)+NH3could be established.

PACS 36.40 – Atomic and molecular clusters.

PACS 34.30 – Intramolecular energy transfer; intramolecular dynamics. PACS 01.30.Cc – Conference proceedings.

1. – Introduction

Recent development of reliable sources for ultrafast laser pulses has stimulated a gen-eration of novel ‘real-time’ studies in chemical reaction dynamics. A couple of years ago A. H. Zewail and coworkers have demonstrated that the chemical reaction of a well-prepared photoexcited molecular state can be followed in a time-resolved experiment [1, 2]. Typi-cally, a pump pulse prepares the system in an electronically excited state which is then probed by a delayed ionizing laser pulse. The resulting ion signal gives a direct measure of the populations of the intermediate reactive state. By variation of the delay between pump and probe laser pulse the time evolution of the reactive state can be observed in real time. From these typical experiments the new field of ‘femtosecond chemistry’ has been developed. An overview of the present state of this field can be found in recent textbooks [3-5].

( 

)Paper presented at the 174. WE-Heraeus-Seminar “New Ideas on Clustering in Nuclear and

Atomic Physics”, Rauischholzhausen (Germany), 9-13 June 1997.

(  

)Also: Fachbereich Physik, Freie Universit¨at Berlin, Arnimallee 14, D-14195 Berlin, Germany

From the various systems studied we have picked two particularly interesting model systems which are somewhat closer to ‘real’ chemistry. Neat ammonia clusters exhibit a prototype internal protonation reaction in electronically excited states. Early photoioniza-tion studies on the nanosecond time scale [6] have shown that the mass spectra are domi-nated by protodomi-nated ions(NH3)

n,2

NH4+in comparison to unprotonated species(NH3)

+

n

. Two mechanisms have been proposed to explain this observation: the protonated cluster ions are formed byNH2 loss either from (NH3)

+

n

after the ionization process by a so-called absorption-ionization-dissociation process (AID) or by fragmentation of the excited clusters(NH3)



n

followed by ionization (ADI mechanism) [7]. The femtosecond pump-probe technique can, in principle, distinguish these two processes: at zero delay times only AID processes are possible, whereas at delay times larger than the lifetime of the excited(NH3)



n

clusters the ion signal arises from an ADI process. Different electroni-cally excited states in the ammonia clusters have been investigated in our group by using different laser wavelengths in UV and VUV spectral range [8-10]. Only very recently we have reinvestigated the lowest electronically excited state (~

Astate) of the ammonia

clusters with additional observation of the kinetic energy of the photoelectrons. Novel information on the internal energy distribution can be extracted from these experiments. We will present some of the recent results in this publication.

As the second system we have investigated the dynamic response of photoexcited sodium ammonia clusters. The complexes are model systems for the solvation of metal atoms in polar solvents. By following the optical properties of these clusters as a func-tion of the number of solvent molecules attached to the sodium atom one can hope to gain detailed information on the solvation process in polar liquids. In our previous studies we have concentrated on the spectroscopic properties such as the ionization potential [11] and the lowest electronically excited state [12-14]. First time-resolved experiments [15] have shown that the lowest electronically excited state of the smallest complexNaNH3, which

asymptotically correlates to the3p-state of the sodium atom, is long-lived (>1 ns), while in

the next larger cluster (Na(NH3)2) the lifetime is already reduced to 30 ps. In addition the

formation of sodium ions has been observed which come from a photo-induced fragmen-tation process. From a careful study of the ion signals as a function of laser wavelengths and intensities we were able to determine the origin of this fragmentation process. The results will be presented here.

2. – Experimental set-ups

The experimental set-ups used in experiments presented here have three major parts: a cluster source, an interaction zone with the laser radiation and a mass selective ion detec-tor. Details have been published elsewhere [8, 16]; here a brief summary will be adequate. The ammonia clusters are formed by an adiabatic expansion of 8% ammonia seeded inHe

gas through a pulsed conical nozzle. Usually(NH3) n,2

NH+4cluster ions upn=10are

ob-served in the time-of-flight (TOF) mass spectrometer. In contrast, a special technique is necessary to produce a beam of sodium-ammonia clusters [17]. In a pickup source a pulsed ammonia beam is crossed with a sodium beam at right angle. TheNa(NH3)

n

clusters are created by collisions between sodium atoms and ammonia molecules. The thus formed complexes are stabilized by additional collisions in the adiabatic expansion. Na(NH3)

n

clusters up ton=80have been observed in the mass spectrum.

The femtosecond laser pulses needed for the time-resolved measurements are pro-duced by a commercial laser system with a regenerative amplifier (Quantronix Model 4810/20) seeded by an Ar-ion laser-pumped Ti : Sapphire laser (Spectra Physics

TSUNAMI). Typical output power is about 300J per pulse with a tuning range from

770 to 840 nm and a pulse duration of 150 fs (FWHM). For the pump-probe experiments the beam is split into two parts. Depending on the type of experiment the output wave-length can be transformed to second, third or forth harmonic by successive BBO crystals positioned in either laser beam. A continuous delay between the two laser beams up to 500 ps can be introduced by a standard delay line.

An important additional experimental tool has been introduced in the experiments with photoexcited ammonia clusters: the observation of the kinetic energy of the pho-toelectrons in correlation to mass of the ionised cluster. This enables, in connection with the femtosecond pump-probe technique, a direct time-resolved observation of the distri-bution of internal energy during and after photoexcitation. A detailed description of this technique will be given elsewhere [18].

3. – Results and discussion

3.1. Ammonia clusters. – For ammonia clusters which represent a prototype system of hydrogen-bonded polar molecules, a particularly large interest exists to study their re-action dynamics by the pump-probe technique with femtosecond (fs) laser pulses. From earlier spectroscopic experiments it is concluded that the ammonia molecule excited to its lowest electronic (~

A) state should decay very fast because of the broad linewidth

mea-sured for this transition. Recent studies of the ammonia clusters(NH3) n

by two-photon resonance ionization with fs laser pulses have revealed ultrafast decay processes after excitation to the state leading mainly to protonated cluster ions(NH3)

n,2

NH+4[7, 8].

We have applied the fs laser system described above for the detailed analysis of the fragmentation and protonation dynamics in excited ammonia molecules and clusters. The fourth harmonic of the Ti:Sapphire laser pulses at1=200nm were used as pump pulses,

whereas the time-delayed probe pulses are given by the second (2 =400nm) or third

(2=267nm) harmonic of the same laser pulses. With a pulse duration of 150 fs we were

able to observe directly, for the first time, the lifetime of the ammonia molecule in the ~ A

state to be only about 40 fs, which corresponds well to the earlier measured linewidth. This lifetime is caused by the ultrafast fragmentation processNH3(

~

A )!NH2+H.

Turning now to the ammonia clusters, a more complex time behaviour is observed as demonstrated in fig. 1. Here the clusters are excited by the pump pulse at 200 nm and ionized by the probe pulse at 400 nm, delayed by the timewith respect to the pump pulse.

On the right-hand side of fig. 1 the unprotonated cluster ions are given, whereas on the left-hand side the protonated ions are shown, formed byNH2fragmentation of the parent

cluster in the ionized as well as in the neutral ~

Astate. The very fast decay of the signals

obtained for =0, i.e. simultaneous irradiation with pump and probe pulses, reflects the

decay of the primarily excited clusters(NH3)nin the ~

Astate with the lifetime20of a few

100 fs. This decay is characterized by ultrafast fragmentation and reorientation (“internal protonation”) of the clusters.

The subsequent decay process with a longer lifetime of about 4ps is due to the fragmen-tation of the secondary populated cluster states in the reoriented configuration. Finally, the time evolution of the clusters leads to long-living (somes) fragment states which

form the nearly constant ion signals for the larger clusters at longer delay times ( 

4 ps). The solid lines in fig. 1 are theoretical curves fitted by our kinetic model. The excellent agreement with the experimental points as well as the identity of the lifetimes

20obtained for the parent ion and its corresponding protonated species demonstrate the

Fig. 1. – Ammonia cluster ion yield as a function of delay timebetween pump ( 1

=200nm) and

probe (2=400nm) laser pulses. Solid lines are model fits.

In a second run of experiments we have detected not only the ammonia cluster ions but also the simultaneously released photoelectrons. For this purpose we have added to the experimental arrangement discussed above a “magnetic bottle” time-of-flight electron spectrometer by means of which the kinetic energy of the photoelectrons can be mea-sured. In this way we obtain information about the energy content in the ion states on the fs time scale from which the energy flow in the excited neutral cluster states can be followed. For the analysis of clusters the coincidence detection of ions and photo-electrons is necessary in order to find the true correlation between a photoelectron and its parent ion. We have combined, for the first time, the coincidence detection of ions and

0 200 400 600 800 (NH3)2NH4 + τ= 0ps 0 10 20 30 40 τ= 4ps 0.0 0.5 1.0 9.02 eV 0 200 400 600 (NH3)3NH4 + τ= 0ps 0 40 80 120 τ= 4ps 0.0 0.5 1.0 8.84 eV 0 100 200 300 electron signal E_el[eV] (NH3)4NH4 + τ= 0ps 0 20 40 60 τ= 4ps 0.0 0.5 1.0 8.78 eV

Fig. 2. – Electron spectra of(NH3)nNH +

4 ,

n =2-4, for =0and =4ps at1 =200nm and 

2

=400nm. The numbers at the arrows represent the appearance potentials of the corresponding

cluster ions.

electrons with the fs pump-probe technique [19].

The application of this method to ammonia molecules and clusters has revealed much deeper insight in the energy variations during the cluster dynamics. In fig. 2, as an example, the electron energy distribution of different cluster sizes is compared for zero delay time and = 4ps. The probe photon wavelength2 = 400nm is identical with

that in fig. 1. At zero delay time the electron spectra extend from zero up to maximum electron energyEelmax =h1+h2,AP, whereh1 =6:1eV andh2 =3:1eV are the

energies of the pump and probe photons, respectively, whereas AP means the appearance potentials (given in fig. 2 by the arrows). The electron signals at = 4ps are nearly

exclusively due to ionization of long-living neutral(NH3) n,2

NH4fragments as discussed

above. Compared to = 0the signals are much smaller (note the different scales) and

quite different in shape. For(NH3)2NH+4the signal is particularly weak at =4ps and

extends only up to aboutEel=0:1eV. Since the ionization potential of(NH3)2NH4(IP =

3.3 eV) is higher than the photon energy (3.1 eV), we have to invoke an internal energy of the(NH3)2NH4fragment of up to 0.3 eV to explain the electron spectrum. For(NH3)3NH4

(IP=2.97 eV) and(NH3)4NH4 (IP=2.73 eV) an internal energy in the fragments of up

to 0.35 eV for(NH3)3NH4 and 0.3 eV for (NH3)4NH4, respectively, is deduced. At low

electron energies (i.e. high internal energy in the ion) the signal is reduced, in particular for(NH3)4NH4, which probably will be caused by the reduced Franck-Condon factors

λ₁= λ₂= 820 nm τ= 70 ps

ion signal

Na

NaNH

delay / ps

ion yield as a function of delay time between pump and probe laser pulses. The signals in the upper row are obtained with pump and probe lasers at the same wavelength (one-colour experiment). In the lower row different wavelengths for the pump laser (

1

=820nm) and

the probe laser (2 =410nm) are used (two-colour experiment).

distribution requires much more information about the spectra and potential curves of the cluster fragment states.

In conclusion the pump-probe technique with fs laser pulses in combination with the coincidence detection of ions and photoelectrons has enabled us to obtain detailed novel information about the fragmentation and protonation dynamics of ammonia clusters.

3.2. Metal ammonia clusters. – To obtain a direct insight in the photo excitation dy-namics of sodium-ammonia complexes we have performed femtosecond pump-probe ex-periments. As one example fig. 3 shows the observedNaNH+3 andNa+ ion signals for

one-colour pump and probe pulses (820 nm, upper row) and two-colour pump and probe pulses (820 nm and 410 nm, lower row). In the two-colour experiment the 410 nm laser pulse follows the 820 nm laser pulse for positive delay times, while for negative delay times the order of the pulses is reversed. In one-colour experiments a symmetrical signal with respect to zero delay time between pump and probe pulse is expected.

TheNaNH+3 ion signal shows a sudden increase at zero delay times and a slow decay

of about 1000 ps for positive delay times in the two-colour pump-probe experiment (lower left part of fig. 3). Under the same experimental conditions an increasingNa+ion signal

-1 0 1 2 3 4 5 Na Na+ Na(NH3) Na(NH3) + 3s 3p 3d 2 A1(3s) 2 E (3p) 2 E (4p) energy / eV dissociation τ= 70 ps

Fig. 4. – Scheme of the energy diagram forNaNH 3and

Nashowing the electronic excited states

involved in the photo excitation and dissociation process. The arrows indicate the photo excitation: short arrows present the laser wavelength of 820 nm, while the long arrows belong to 410 nm pho-tons.

and probe laser pulses theNaNH+3ion signal decays with a time constant of 70 ps while

theNa+ion signal rises with the same time constant (upper row of fig. 3).

To understand the observed signals spectroscopic data of theNaNH3complex have to

be known. Figure 4 shows the relevant electronically excited states ofNaandNaNH3

schematically. NaNH3has a strong absorption band near 820 nm which originates from

the3s! 3ptransition of the Na atom shifted to the red due to the interaction with the

ammonia molecule [12]. Thus, as the first step in both experimental situations the pump pulse populates the electronically excited2E(3p)state resonantly. In the two-colour

ex-periment the probe laser pulse at 410 nm directly ionisesNaNH3leading to the strong ion

signal observed for positive delay times. The slow exponential decay of the excited state with a time constant of about 1000 ps is significantly faster than the expected radiative deexcitation which is known to be 16 ns for a free sodium atom. This leads to the conclu-sion that the2E(3p)state undergoes a slow non-radiative decay, possibly a fragmentation Na(3p)NH3 ! Na( 3s)+NH3 which for energetical reasons can only produce sodium

atoms in the ground state. Thus, this process cannot explain the increasingNa+ion

sig-nal observed in the two-colour experiment since the observed time constant is much too fast (75 ps) and the photon energy of the probe pulse (410 nm) is to small to ioniseNa(3s)

atoms.

If one takes an absorption of two photons of the pump laser pulse into account, which is possible due to the high intensities of the femtosecond laser pulses and the resonant intermediate state, the observed sodium ion signal can be understood as follows. A small portion of theNaNH3complexes is excited into the2E(4p)state by the pump laser pulse

excited sodium atom is ionised with the probe laser pulse at 410 nm as indicated in fig. 4. To prove this assumption we have conducted a one-colour pump-probe experiment. In this case the strong ion signal of the sodium-ammonia complex can be suppressed and the time evolution of theNaNH+3ion signal can be observed directly. As shown in fig. 3 (upper

row) theNaNH+3ion signal decays with a time constant of 70 ps, while theNa+ion signal

increases with the same time constant. The observation of theNa+signal results from a

two-photon process with a resonant3p ! 3dtransition as indicated in fig. 4. The

iden-tical time constants for the observedNa+ signal in the one- and two-colour experiments

justify the assumption that in both cases the decay of2E(4p)state is observed. Further

evidence for the assumed process has been found from the wavelength dependence of the ion signals.

In conclusion, by comparison of the one- and two-colour pump-probe experiments the energy flow after photo excitation of NaNH3 complexes could be disentangled. While

the first electronically excited2E(3p)state decays on a rather long time scale of about

1000 ps, the higher excited state undergoes a dissociation processNaNH3 ,

2E(4p)

!

Na(3p)+NH3within 70 ps.

An important question at this stage is the mechanism of the decay process. Theoretical calculations of the potential energy surfaces in one dimension [20, 21] have not found any evidence for a dissociative curve crossing of excited states. Multi-dimensional potential energy surfaces for theNaNH3system are urgently needed but presently not available.

From the experimental side the replacement ofNH3by the deuterated counterpartND3

may lead to additional information about the decay mechanism and the vibrational modes involved in the dissociation process. Experiments of this type have been conducted in our laboratory lately and will be published elsewhere [22].

4. – Conclusions

Photon-induced dynamics were studied in two molecular systems. In the ammonia clusters, which present a prototype hydrogen-bonded molecular complex, ultrafast rear-rangement and fragmentation on the time scale of a few 100 fs are observed for the lowest electronically excited ~

Astate. The former process can be understood as an internal

pro-tonation within an ammonia dimer subsystem

 (NH3)2 h !NH4NH2  . With an advanced detection technique, where mass- and time-resolved photoelectron spectra are recorded, the internal energy flow in excited ammonia clusters and their fragments can be followed. The analysis of one- and two-colour pump-probe experiments ofNaNH3 complexes

have shown that the complex dissociates from a higher electronically excited state2E(4p)

within 70 ps. The experiments reveal that sodium atoms in the3pstate are formed during

the dissociation process. The decay mechanism is still under question. The rather long decay time of 70 ps indicates that the dissociation either is hindered by a barrier or the interaction with the repulsive state is weak. Multi-dimensional potential energy surfaces which can elucidate the process are presently not available.

  

The authors would like to thank Dr. V. STERT, Dr. T. FREUDENBERG, Dr. J.

H¨OHNDORF and A. SCHOLZ for their great effort in the ammonia clusters and in the sodium-ammonia experiments. This work has been financially supported by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 337, Projects A9 and A11.

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