FORMATION AND LUMINESCENCE OF MOLYBDENUM ATOMS AFTER UV MULTIPHOTON EXCITATION OF GAS PHASE Mo ( CO ) 6

Photodissociation of metal carbonyls in gas phase under UV radiation has been extensively studied for a long time. In one of the first papers on this subject it was shown that Fe(CO)5 molecule can lose its CO ligands under UV excitation, The investigation of UV multiphoton dissociation (MPD) of organometallic molecules shows that MPD of practically any of such molecules yields mainly metal atoms or ions. For example, the excitation of gas-phase Mo(CO)6 by UV laser radiation (2 193 nm, 248 nm, 351 nm) with a low fluence (about several mJ/cm2) gives mainly Mo(CO)5, Mo(CO)4, Mo(CO)3.3’4 As the laser fluence increases, radicals with a smaller number of CO ligands and Mo atoms begin to appear among the photodissociation products. At higher fluences (hundreds of mJ/cm2) Mo ions are formed. The formation of Mo atoms in different states under the KrF laser excitation of the Mo(CO)6 (2 248 nm) with a moderate fluence (5 to 40mJ/cm2) was observed in Reference 6. The population distribution of the atoms over the excited states in the energy range from 25000 to 35000 cmwas found to be Boltzmann with a characteristic temperature of 11000 + 2500 K. The luminescence of Mo atoms from different energy levels was observed after excitation of gas-phase Mo(CO)6 by XeC1 laser radiation ( 100 mJ/cm2). The interesting feature of this luminescence is appearance of the delayed atomic emission, along with the well known luminescence which occurs simultaneously with the laser pulse. The time delay between the laser pulse and the maximum of the delayed luminescence pulse is about a few hundreds of nanoseconds. We found several transitions from highly excited atomic levels which result in delayed Mo emission after XeC1 laser excitation of Mo(CO) 6.


INTRODUCTION
Photodissociation of metal carbonyls in gas phase under UV radiation has been extensively studied for a long time. In one of the first papers on this subject it was shown that Fe(CO)5 molecule can lose its CO ligands under UV excitation, The investigation of UV multiphoton dissociation (MPD) of organometallic molecules shows that MPD of practically any of such molecules yields mainly metal atoms or ions. 2 For example, the excitation of gas-phase Mo(CO)6 by UV laser radiation (2 193 nm, 248 nm, 351 nm) with a low fluence (about several mJ/cm2) gives mainly Mo(CO)5, Mo(CO)4, Mo(CO)3 .3'4 As the laser fluence increases, radicals with a smaller number of CO ligands and Mo atoms begin to appear among the photodissociation products. At higher fluences (hundreds of mJ/cm2) Mo / ions are formed. 5 The formation of Mo atoms in different states under the KrF laser excitation of the Mo(CO)6 (2 248 nm) with a moderate fluence (5 to 40mJ/cm2) was observed in Reference 6. The population distribution of the atoms over the excited states in the energy range from 25000 to 35000 cmwas found to be Boltzmann with a characteristic temperature of 11000 + 2500 K.
The luminescence of Mo atoms from different energy levels was observed after excitation of gas-phase Mo(CO)6 by XeC1 laser radiation ( 100 mJ/cm2). 7 The interesting feature of this luminescence is appearance of the delayed atomic emission, along with the well known luminescence which occurs simultaneously with the laser pulse. The time delay between the laser pulse and the maximum of the delayed luminescence pulse is about a few hundreds of nanoseconds. We found several transitions from highly excited atomic levels which result in delayed Mo emission after XeC1 laser excitation of Mo (CO) 6. 224 YU. E. BELYAYEV et al.
In this paper we present our study of the delayed atomic emission which appeared at the zTP -eTD transition of Mo under XeC1 laser excitation of Mo(CO)6 gas.

EXPERIMENTAL
The experimental setup comprised a XeCl-laser (2 308 nm, pulse width z 15 ns (FWHM), energy E 50mJ), a stainless-steel vacuum chamber (120 x 350 x 350 mm) having quartz windows ( 60 mm), a monochromator (slit spectral width 20/l/mm) and a gated optical multichannel analyzer (OMA). The spectral resolution was 2/I,. The temporal evolution of luminescence at a certain fixed wavelength was studied using a photomultiplier (spectral response 200 to 800 nm). The laser radiation was focused into the chamber with a quartz lens (f 20 cm), the laser fluence in the observation region was about 300 mJ/cm 2. The luminescence signal was accumulated with its subsequent averaging, by using the digital oscilloscope and a computer.
The vacuum chamber was evacuated to 10-5 Torr with an oil-diffusion pump fitted with a liquid-nitrogen trap. The Mo(CO)6 pressure range used was 0.02 to 0.1 Torr.
The pressure of the buffer gas was varied between 0.01 and 500 Torr.
The Mo(CO )6 carbonyl was commercial-grade and was sublimed and stored under vacuum before being used.

RESULTS
A large number of atomic Mo lines are observed in the luminescence spectrum when Mo(CO)6 is excited by XeCl-laser with the fluence = 300mJ/cm 2. The luminescence of Mo2 dimers is also presented in the spectrum.
Delayed atomic lines present a special interest. These are atomic lines of Mo whose intensity reaches a maximum in several hundreds of nanoseconds after a laser pulse and then slowly, in several microseconds, decreases ( Figure 1 ). Delayed luminescence was studied for the'transitions z7P4 . ,--e7D3_5, the most intense line corresponds to the transition zTP --eTDs. This transition is started at an energy of 44970.10 cmand is terminated at 26320.38 cm-1. The radiative time for these transitions is only of the order of ten nanoseconds. It should be noted that there was not any distinct luminescence from levels close to the eTDj level.
The tail of the luminescence pulse ( Figure 1 shows a near exponential decay. The decrease of the tail emission intensity can be characterized by a single decay time, ZD. This is the time interval when the tail intensity drops from 1 to 1/e. To characterize the whole pulse of the delayed atomic emission we also used time integrated luminescence intensity, lin Figure  The time-integrated intensity, lint, and the luminescence decay time, "D, were measured as a function of buffer gas pressure. It has been found that with addition of noble gases (He, Ne, Ar, Kr, Xe) Iin increases and then drops almost to zero due to collisional relaxation. The decay time, ZD, behaves in a similar way. The buffer gas pressure corresponding to maximum of lin and ZD decreases with atomic mass of the buffer gas. The only exception is Ne since the maximum of Iin and ZD for this buffer is reached at higher pressures, about 100 Torr.
We studied also the influence of the electronegative gases CC14 and SF 6 on the delayed luminescence. Figure 4 shows the dependence of Iin on their pressure. The integral luminescence drops rapidly with the pressure of SF 6 and CC14, starting with very low pressures: 2.10 -3 Torr for CCI, and 3.10 -2 Torr for SF 6 ( Figure 4). The pressure of Mo(CO)6 in this case was 0.04 Torr.  (2), which are able to recombine when colliding with the parent molecule. The recombination process produces Rydberg atoms Mo** (2).
The radiative decay of Rydberg atoms populates the eTD3_5 atomic state and gives rise to delayed atomic emission at the transition zTP eTD3_5.
Finally, the laser radiation gives rise to highly excited Rydberg atoms in various states which can be divided into two groups" A1* are Rydberg atoms prepared in the states the direct radiative transitions from which to the eTDj state are forbidden and A* are Rydberg atoms in the levels the transitions from which to the eTDj state are allowed and relatively fast (small principal quantum number n). Let us suppose that the fraction of A* atoms is very low compared to At'*. As a result of their collisions with the buffer gas or with parent molecules Mo(CO)6, the Rydberg atoms AI'* transfer to A*. The transition to the Rydberg states, A*, may occur both due to relaxation with a change in the main quantum number n and due to the mixing  of states with different quantum numbers and the same energies, a Then A* decay to the eTDj state, and the radiative transition from this state to zTP is recorded as delayed luminescence. The subsequent transition to the ground state aTS 3 from zTP can be also observed in the luminescence spectrum.
The dependence of integral luminescence intensity, Iint, on Mo(CO)6 pressure should be linear for direct production of Rydberg atoms 1 or quadratic for creation of these atoms in the recombination process (2). The experimental results give the slope 1.8 +_ 0.5. It means that recombination mechanism (2) produces Mo Rydberg atoms.
The radiative lifetime of the eTO --zTP transition is only several nanoseconds and is much shorter than the characteristic times of delayed luminescence. Therefore, the time evolution of delayed luminescence is defined by the population kinetics of the A* Rydberg states. The leading edge of the luminescence pulse is determined by the transitions of atoms to the A* states, and the trailing edge depends on the lifetime of these states. The Rydberg state lifetime can be estimated from Reference 9. The decay time Zo 1.5 #S corresponds to the lifetime of Rydberg atoms in the state with the principal quantum number n being equal to 11. As the Mo(CO)6 pressure rises or some buffer gas is added, the transitions from A]'* to A* states are accelerated FORMATION AND LUMINESCENCE OF MOLYBDENUM ATOMS 229 which makes the leading edge shorter. The initial increase of integral luminescence, as the buffer gas pressure rises, can be apparently explained by collisional transitions to A* from other Rydberg states. A further increase in buffer gas pressure makes the collisional relaxation of A* state faster which leads to luminescence quenching with higher buffer gas pressures.
An anomalously weak influence of Ne gas on delayed luminescence (Figure 3 can be explained also in the frame of the processes (1)-(3). It is well known that the Ne scattering length for thermal electrons is smaller than that for other noble gases. 9 The scattering of thermal electrons is similar to those of Rydberg atoms since the electron in a Rydberg atom is far from the nucleus and hence is loosely bound to it.
The experiments with electronegative gases also confirm the Rydberg atoms hypothesis. When the SF 6 and CCI molecules collide with a Rydberg atom, they trap the Rydberg electron and ionize the atom. The rapid decrease of integral luminescence with the pressure of electronegative gases points to the presence of such a process ( Figure 4). The effect of CCI on luminescence begins to manifest itself at partial pressures lower than in the case of SF6, which seems to be connected with a higher value of the electron affinity (2.12 eV for CC1 and 1.48 eV for SF6).1

CONCLUSION
The delayed atomic luminescence which occurs after excitation of gas-phase Mo(CO)6 by XeC1 laser can be explained by the production of Mo Rydberg atoms. Laser radiation gives rise to Rydberg Mo atoms that via the relaxation process populate the starting eTD states. The optical transition from these states is observed as delayed luminescence. The long luminescence tail is caused by relatively long lifetimes of Rydberg atoms.