DYNAMICS OF NEOPENTANE IN DENSE ARGON GASES

By means of inelastic neutron scattering we measure the dynamic struct ure factors S(k,omega) of three dense mixtures of neopentane and Ar-40 (invisible for neutrons) at 323 K for wave numbers 4<k<12 nm(-1) and partial neopentane number densities n(neo)=0.4, 2.7, and 3 nm(-3). For the first two samples, with deuterated neopentane C(CD3)(4), S(k,omeg a) can be described by one Lorentzian in frequency omega. For k<5 nm(- 1) the half-width at half maximum is omega(H)(k)=D(c)k(2), with D-c=21 and 13X10(-9) m(2)s(-1), the collective translational diffusion coeff icients far neopentane at n(neo)=0.4 and 2.7 nm(-3), respectively. For k>6 nm(-1) and n(neo)=2.7 nm(-3), omega(H)(k) shows a behavior remini scent of ''de Gennes narrowing.'' For n(neo)=3 nm(-3), with C(CH3)(4), the (incoherent) S(k,omega) can be described with two Lorentzians, du e to translational and rotational motions of single neopentane molecul es, yielding the self-diffusion coefficient D-s=9X10(-9) m(2)s(-1) and the rotational diffusion coefficient D-r=0.8+/-0.3 ps(-1). The result s are consistent with kinetic theory calculations.