Light at optical frequencies interacting with a metal surface can excite interband quantum transitions, or intraband currents at frequencies approaching the PHz range. Momentum conservation enables the interband excitation to occur in first order as a dipole transition, while intraband excitations involve second-order momentum scattering processes. The free electron response to optical fields can also be collective, causing the optical field to be screened by the multipole plasmon response. We describe the exitation of single crystal silver surfaces in the region where the dielectric response transits from negative to positive passing through the epsilon near zero (ENZ) condition. There, electrons can no longer screen the optical field, so that it penetrates as a collective charge density wave of the free electron plasma, in other words, as a bulk transverse or longitudinal plasmon field. We examine two-photon photoemission (2PP) signals from Ag(1 1 1), (1 0 0) and (1 1 0) surfaces through the ENZ region under conditions where intraband, and interband single particle, and bulk plasmon collective responses dominate. We are specifically interested in the bulk plasmon decay into plasmonic photoemission. Plasmonic decay into excitation of electrons from the Fermi level, which we observe as a nonlinear 2PP process, has been established for the free electron and noble metals, but its significance to transduction of optical-to-electronic energy has not penetrated the plasmonics community. 2PP spectra show evidence for intraband hot electron generation, interband surface and bulk band excitation, and nonlinear bulk plasmon driven plasmonic single particle excitation. Because the intraband and plasmonic decay into hot electron distributions have been extensively considered in the literature, without reference to explicit experimental measurements, we discuss such processes in light of the directional anisotropy of the electronic structure of single crystalline silver. We note that projected band gaps in silver exclude large regions of the unoccupied state density from hot electron generation, such that it predominantly occurs in the (1 1 0) direction. Moreover, the excited hot electron distributions do not follow expectations from the joint density of the occupied and unoccupied states of a free electron metal, as assumed in majority of research on hot electron processes. We describe the strongly anisotropic hot electron distributions recorded by 2PP spectroscopy of Ag surfaces, and the plasmonic photoemission process that occurs on all surfaces irrespective of the momentum-dependent single particle band structure of silver. Plasmonic photoemission, or its linear analog that excites hot electrons at energies below the work function of Ag, is an important process for harvesting hot electron energy in photocatalytic and electronic device applications because the plasmon energy is not distributed between an electron and hole. This plasmonic decay channel is robust, but many aspects raise further questions. The accompanying publication by Gumhalter and Novko discusses the plasmonic photoemission from a theoretical point of view and its extension to Floquet engineering, as an exploration of novel plasmonic excitation processes in metals.

光频率的光与金属表面相互作用可以激发带间量子跃迁，或接近 PHz 范围频率的带内电流。动量守恒使得带间激励能够以偶极子跃迁的形式发生，而带内激励则涉及二阶动量散射过程。自由电子对光场的响应也可以是集体的，导致光场被多极等离子体响应屏蔽。我们描述单晶银的退出介电响应通过 epsilon 近零 (ENZ) 条件从负转变为正的区域中的表面。在那里，电子不再能屏蔽光场，因此它作为自由电子等离子体的集体电荷密度波渗透，换句话说，作为体横向或纵向等离子体场。 我们在带内、带间单粒子和体等离子体集体响应占主导地位的条件下，检查来自 Ag(11 1)、(1 0 0) 和 (1 1。我们特别感兴趣的是体等离激元衰变成等离激元光发射。等离子体衰变为电子从费米能级的激发 ，我们将其视为非线性 2PP 过程，已针对自由电子和贵金属建立，但其对光电能量转换的重要性尚未渗透到等离子体激元界。2PP 谱显示了带内热电子生成、带间表面和体带激发以及非线性体等离子体驱动的等离子体单粒子激发的证据。由于带内和等离子体衰变成热电子分布已在文献中得到广泛考虑，而没有参考明确的实验测量，因此我们根据单晶银电子结构的方向各向异性讨论此类过程。我们注意到，银中的预计带隙排除了热电子产生中的大部分未占据态密度区域，因此它主要发生在（1  1 0) 方向。此外，激发的热电子分布并不遵循自由电子金属占据和未占据状态的联合密度的预期，正如大多数热电子过程研究中所假设的那样。我们描述了银表面 2PP 光谱记录的强各向异性热电子分布，以及发生在所有表面上的等离激元光电子过程，无论银的动量依赖性单粒子能带结构如何。等离子体光发射或其以低于银功函数的能量激发热电子的线性模拟，是光催化和电子设备应用中收集热电子能量的重要过程，因为等离子体能量不分布在电子和空穴之间。这种等离子体衰变通道是强大的，但许多方面提出了进一步的问题。Gumhalter 和 Novko 的随附出版物从理论角度讨论了等离子体光电子发射及其对 Floquet 工程的扩展，作为对金属中新型等离子体激发过程的探索。