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Last Time MatterMatter-Energy interactions are problematic – Matter = collection of oscillating dipoles – High ν oscillators “missing in action” at low T Quantized Energy Classical energy exchange q1 q2 1 p2 PE = KE = m v2 = R 2 2m ΔE Æ 0 Quantized energy exchange ΔE = h ν Photoelectric Effect Monochromatic light ejects electrons from metal surface Measure #electrons (current) Measure electron KE by adjusting voltage (QM if hν/kT not << 1) Photoelectric Effect Classical physics says – Elight proportional to intensity (not frequency) frequency) Predicts – If electrons bound to atoms, Intensity threshold for current Intensity and electron KE rise together Current & KE do not depend on frequency Observe the opposite! 1 Photoelectric Effect Atomic Spectra Observations – Current rises with intensity no threshold – KE = hν − φ KE Indep’t of intensity Einstein (1879(1879-1955) ν – 1905 – light energy = photons Atomic Spectra Magic numbers predict H atom λ 1 1 y j z − = RH i 2 λ n2 { k n1 1 Why? where n > n1 and are integers Atomic Spectra Bohr (1885 – 1962) – 1913, H atom model electron in stable circular orbit Light emitted if electron jumps orbits – hi E orbit Æ lo E orbit – E change = hν Angular momentum quantized = nh/2π nh/2π – n is (magic) quantum number Rydberg RH = 2 π 2 me q e 4 h3 c 2 Sporadic Progress Heat capacity Blackbody, 1900 Æ E quantized Photoelectric, 1905 Æ E localized Atomic spectra, 1913 Æ Connects quantum theory to huge volume of data What about matter? de Broglie’s Proposal de Broglie (1892(1892-1987) 1923 – photons E = h ν = m c2 h / λ = m c = momentum – matter h/λ=mv=p – de Broglie λ=h/p Why do atoms exist? de Broglie’s wavelength m = relativistic photon mass (rest mass = 0) Measure λ by Diffraction “quick” electron me = 9.1 x 10-31 kg v = 3 x 107 m/s λ = 24 pm ave Ar atom at 298 K m = 6.63 x 10-26 kg v = (3 k T / m)1/2 = 430 m/s λ = 23 pm Alan walking (88 kg)(1 m/s) m/s) λ = 7.6 x 10-24 pm 3 Interference 4