  The energy-level structure of a quantum system plays a fundamental role in
determining its behavior and manifests itself in a discrete absorption and
emission spectrum. Conventionally, spectra are probed via frequency
spectroscopy whereby the frequency \nu of a harmonic driving field is varied to
fulfill the conditions \Delta E = h \nu, where the driving field is resonant
with the level separation \Delta E (h is Planck's constant). Although this
technique has been successfully employed in a variety of physical systems,
including natural and artificial atoms and molecules, its application is not
universally straightforward, and becomes extremely challenging for frequencies
in the range of 10's and 100's of gigahertz. Here we demonstrate an alternative
approach, whereby a harmonic driving field sweeps the atom through its
energy-level avoided crossings at a fixed frequency, surmounting many of the
limitations of the conventional approach. Spectroscopic information is obtained
from the amplitude dependence of the system response. The resulting
``spectroscopy diamonds'' contain interference patterns and population
inversion that serve as a fingerprint of the atom's spectrum. By analyzing
these features, we determine the energy spectrum of a manifold of states with
energies from 0.01 to 120 GHz \times h in a superconducting artificial atom,
using a driving frequency near 0.1 GHz. This approach provides a means to
manipulate and characterize systems over a broad bandwidth, using only a single
driving frequency that may be orders of magnitude smaller than the energy
scales being probed.
