We study the optical excitation spectrum of an atom in the vicinity of a dielectric surface. We calculate the rates of the total scattering and the scattering into the evanescent modes. With a proper assessment of the limitations, we demonstrate the portability of the flat-surface results to an experimental situation with a nanofiber. The effect of the surface-induced potential on the excitation spectrum for free-to-bound transitions is shown to be weak. On the contrary, the effect for

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... bound transitions is significant leading to a large excitation linewidth, a substantial negative shift of the peak position, and a strong long tail on the negative side and a small short tail on the positive side of the field-atom frequency detuning. The study of individual neutral atoms in the vicinity of dielectric and metal surfaces has gained renewed interest due to progress in atom optics and nanotechnology [1, 2, 3, 4, 5, 6, 7] . The ability of manipulating atoms near surfaces has established them as a tool for cutting-edge applications such as quantum computation [8, 9, 10] , atom chips [11, 12] , and probes, very sensitive to the surface-induced perturbations [13] . Recently, translational levels of an atom in a surface-induced potential have been studied [1, 2, 3, 4 ]. An optical technique for loading atoms into quantum adsorption states of a dielectric surface has been suggested [1, 2]. An experimental observation of the excitation spectrum of cesium atoms in quantum adsorption states of a nanofiber surface has been reported [3] . Spontaneous radiative decay of translational levels of an atom near a dielectric surface has been investigated [4] . In this paper, we study the optical excitation spectrum of an atom in a surface-induced potential. We assume the whole space to be divided into two regions, namely, the half-space x < 0, occupied by a nondispersive nonabsorbing dielectric medium (medium 1), and the half-space x > 0, occupied by vacuum (medium 2). We examine an atom, with an upper internal level e and a lower internal level g, moving in the empty half-space The potential energy of the surface-atom interaction is a combination of a long-range van der Waals attraction −C 3 /x 3 and a short-range repulsion [14] . Here C 3 is the van der Waals coefficient. We approximate the shortrange repulsion by an exponential function Ae −αx , where the parameters A and α determine the height and range, respectively, of the repulsion. The combined potential depends on the internal state of the atom (see Fig. 1 ), and is presented in the form V j (x) = A j e −αjx − C 3j /x 3 , where j = e or g labels the internal state of the atom. The potential parameters C 3j , A j , and α j depend on the dielectric and the atom. In numerical calculations, we use the parameters of fused silica, for the dielectric, and the parameters of atomic cesium with the D 2 line, for the two-level atomic model. According to Ref. [4], the parameters of the ground-and excited-state potentials for the silica-cesium interaction are C 3g = 1.56 kHz µm 3 , C 3e = 3.09 kHz µm 3 , A g = 1.6×10 18 Hz, A e = 3.17×10 18 Hz, and α g = α e = 53 nm −1 . We introduce the notation ϕ a ≡ ϕ νe and ϕ b ≡ ϕ νg for the eigenfunctions of the center-of-mass motion of the atom in the potentials V e and V g , respectively. They are determined by the stationary Schrödinger equations (1) Here m is the mass of the atom. The eigenvalues E a ≡ E νe and E b ≡ E νg are the total center-of-mass energies of the translational levels of the excited and ground states, respectively. These eigenvalues are the shifts of the energies of the translational levels from the energies of the corresponding internal states. Without the loss of generality, we assume that the center-of-mass eigenfunctions ϕ a and ϕ b are real functions. FIG. 1: Energies and wave functions of the center-of-mass motion of the ground-and excited-state atoms in the surfaceinduced potentials. The parameters of the potentials are C3g = 1.56 kHz µm 3 , C3e = 3.09 kHz µm 3 , Ag = 1.6 × 10 18 Hz, Ae = 3.17 × 10 18 Hz, and αg = αe = 53 nm −1 . The mass of atomic cesium m = 132.9 a.u. is used. We plot, for the excited state, two bound levels (ν = 400 and 415) and, for the ground state, two bound levels (ν = 281 and 285) and one free level (E f = 4.25 MHz). We introduce the combined eigenstates |a = |e ⊗ |ϕ a and |b = |g ⊗ |ϕ b , which are formed from the internal and translational eigenstates. The corresponding energies arehω a =hω e + E a andhω b =hω g + E b . Here, ω j with j = e or g is the frequency of the internal level j. Then, the Hamiltonian of the atom moving in the surfaceinduced potential can be represented in the diagonal form H A = ah ω a |a a| + bh ω b |b b|. We emphasize that the summations over a and b include both the discrete